Struggling With a Basic Question in MS Access - I'm New [duplicate]

Struggling With a Basic Question in MS Access - I'm New [duplicate] - ms-access

I'm new to SQL and try to get some handy knowledge from the book "SQL for Microsoft Access 2nd Edition" published in 2008.
In chapter 3, keyword ON UPDATE CASCADE ON DELETE CASCADE are introduced. I tried to run the statements with the keywords in MS Access 2013's SQL view. There is an error message saying
"Syntax error in CONSTRAINT clause."
The statements work perfectly without ON UPDATE CASCADE ON DELETE CASCADE.
The note in the book explain the keywords don't work on the version before SQL-92. I guess Access 2013 is way after SQL-92.
Can anyone explain to me why the keywords don't work?
below is the statements (ON UPDATE CASCADE ON DELETE CASCADE is at the end):
CREATE TABLE tblManufacturers
(
ManufacturerID INTEGER CONSTRAINT ManfID PRIMARY KEY,
ToyID INTEGER NOT NULL,
CompanyName CHAR (50) NOT NULL,
Address CHAR (50) NOT NULL,
City CHAR (20) NOT NULL,
State CHAR (2) NOT NULL,
PostalCode CHAR (5) NOT NULL,
AreaCode CHAR (3) NOT NULL,
PhoneNumber CHAR (8) NOT NULL UNIQUE,
CONSTRAINT ToyFk FOREIGN KEY (ToyID) REFERENCES tblToys (ToyID)
ON UPDATE CASCADE
ON DELETE CASCADE
);

Those keywords don't work because DAO doesn't support them. Built-in query builder also uses DAO. If you want to create table using CASCADE keywords, it can be done in VBA using ADO only:
CurrentProject.Connection.Execute strSQL
strSQL here contains CREATE TABLE statement

The ON UPDATE CASCADE and ON DELETE CASCADE statements are not supported by Access (see https://msdn.microsoft.com/en-us/library/office/ff836971.aspx).
Regarding the functionality, you should not have a need to use ON UPDATE CASCADE. This constraint means that if you do change a primary key in the master table, the changes propagate to every child table referencing the master table. Changing a primary key is considered a no-go in the SQL world. Is you really must change primary keys (because of some kind of disaster struck), this will be done with a script, lots of backup and extreme care. A primary key is this: a unique (ideally globally), immutable identifier of a row in a table.
The ON DELETE CASCADE means that if you remove a row in the master table, any rows referencing this key in child tables will also be deleted. While this sounds like a lazy shortcut, I would recommend against it and doing this within application logic (there might be a use case where you want to retain records or log them or do something with them instead of blindly erasing them from the database.

Related

This table does not contain a unique column. Grid edit, checkbox, Edit, Copy and Delete features are not available, 2020

Why does phpMyAdmin give me this warning , and lack of function, when selecting from a table named only in lowercase (and underscores) that does have a (single column) primary key? I checked these elements after seeing this
Specifically my query is
SELECT su.* FROM `r8u2d_comps_testsubitem` su
JOIN `r8u2d_comps_testitem` ti ON ti.id=su.testitemid
JOIN `r8u2d_comps_test` t ON ti.testid=t.id
WHERE t.id=241
ORDER BY ti.ordering
The table aliased as "su" has a column "id" (int(11), autoincrement) and a primary key using only this field. It looks to me like this query avoids all the restrictions listed in this answer, so what's the problem? Is it phpMyAdmin (my hosting company has 4.7.9, but I get the same problem locally with 5.0.4) or MySQL (host has 5.7.29-0ubuntu0.16.04.1 - (Ubuntu), I have 10.4.17-MariaDB - MariaDB Server, not strictly comparable I suppose).
Table structure
`id` INT NOT NULL AUTO_INCREMENT,
`testitemid` INT NOT NULL
`marker` CHAR(20) NULL
`text` TEXT NOT NULL,
`ordering` TINYINT NOT NULL,
PRIMARY KEY (`id`),
KEY `testitemid` (`testitemid`),
KEY `ordering` (`ordering`),
CONSTRAINT `subelementToElement`
FOREIGN KEY (`testitemid`) REFERENCES `#__comps_testitem`(`id`)
ON DELETE CASCADE
ON UPDATE NO ACTION

phpMyAdmin makes an effort to work with a primary/unique key for the purposes of enabling grid editing, but that detection logic doesn't hold up very well when using with multiple JOIN statements. It gets difficult for the phpMyAdmin parser to work backwards through some queries and determine which columns come from which tables and whether there's a primary key that could be used for editing the data. I suppose the warning message could better be written as something like "This table or query does not contain a unique column, or your query is a join that obfuscates the original table structure enough that we don't want to risk damaging your data."
Unfortunately, aside from someone rewriting this part of phpMyAdmin, the best solution I can recommend right now is to find the data you want to modify through your JOIN query then open that individual table and scroll through the Browse view to (or use Search to find) the row you wish to modify from the table directly.

Error 1215 while creating foreign key on varchar column [duplicate]

I am trying to forward engineer my new schema onto my database server, but I can't figure out why I am getting this error.
I've tried to search for the answer here, but everything I've found has said to either set the database engine to InnoDB or to make sure the keys I'm trying to use as a foreign key are primary keys in their own tables. I have done both of these things, if I'm not mistaken. What else can I do?
Executing SQL script in server
ERROR: Error 1215: Cannot add foreign key constraint
-- -----------------------------------------------------
-- Table `Alternative_Pathways`.`Clients_has_Staff`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Clients_has_Staff` (
`Clients_Case_Number` INT NOT NULL ,
`Staff_Emp_ID` INT NOT NULL ,
PRIMARY KEY (`Clients_Case_Number`, `Staff_Emp_ID`) ,
INDEX `fk_Clients_has_Staff_Staff1_idx` (`Staff_Emp_ID` ASC) ,
INDEX `fk_Clients_has_Staff_Clients_idx` (`Clients_Case_Number` ASC) ,
CONSTRAINT `fk_Clients_has_Staff_Clients`
FOREIGN KEY (`Clients_Case_Number` )
REFERENCES `Alternative_Pathways`.`Clients` (`Case_Number` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_Clients_has_Staff_Staff1`
FOREIGN KEY (`Staff_Emp_ID` )
REFERENCES `Alternative_Pathways`.`Staff` (`Emp_ID` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB
SQL script execution finished: statements: 7 succeeded, 1 failed
Here is the SQL for the parent tables.
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Clients` (
`Case_Number` INT NOT NULL ,
`First_Name` CHAR(10) NULL ,
`Middle_Name` CHAR(10) NULL ,
`Last_Name` CHAR(10) NULL ,
`Address` CHAR(50) NULL ,
`Phone_Number` INT(10) NULL ,
PRIMARY KEY (`Case_Number`) )
ENGINE = InnoDB
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Staff` (
`Emp_ID` INT NOT NULL ,
`First_Name` CHAR(10) NULL ,
`Middle_Name` CHAR(10) NULL ,
`Last_Name` CHAR(10) NULL ,
PRIMARY KEY (`Emp_ID`) )
ENGINE = InnoDB

I'm guessing that Clients.Case_Number and/or Staff.Emp_ID are not exactly the same data type as Clients_has_Staff.Clients_Case_Number and Clients_has_Staff.Staff_Emp_ID.
Perhaps the columns in the parent tables are INT UNSIGNED?
They need to be exactly the same data type in both tables.

Reasons you may get a foreign key constraint error:
You are not using InnoDB as the engine on all tables.
You are trying to reference a nonexistent key on the target table. Make sure it is a key on the other table (it can be a primary or unique key, or just a key)
The types of the columns are not the same (an exception is the column on the referencing table can be nullable even if it is not nullable in the referenced table).
If the primary key or foreign key is a varchar, make sure the collation is the same for both.
One of the reasons may also be that the column you are using for ON DELETE SET NULL is not defined to be null. So make sure that the column is set default null.
Check these.

For others, the same error may not always be due to a column type mismatch. You can find out more information about a MySQL foreign key error by issuing the command
SHOW ENGINE INNODB STATUS;
You may find an error near the top of the printed message. Something like
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.

Error 1215 is an annoying one. Explosion Pill's answer covers the basics. You want to make sure to start from there. However, there are more, much more subtle cases to look out for:
For example, when you try to link up PRIMARY KEYs of different tables, make sure to provide proper ON UPDATE and ON DELETE options. E.g.:
...
PRIMARY KEY (`id`),
FOREIGN KEY (`id`) REFERENCES `t` (`other_id`) ON DELETE SET NULL
....
won't fly, because PRIMARY KEYs (such as id) can't be NULL.
I am sure, there are even more, similarly subtle issues when adding these sort of constraints, which is why when coming across constraint errors, always make sure that the constraints and their implications make sense in your current context. Good luck with your error 1215!

Check the collation of the table. Using SHOW TABLE STATUS, you can check information about the tables, including the collation.
Both tables have to have the same collation.
It's happened to me.

In my case, I had deleted a table using SET FOREIGN_KEY_CHECKS=0, then SET FOREIGN_KEY_CHECKS=1 after. When I went to reload the table, I got error 1215. The problem was there was another table in the database that had a foreign key to the table I had deleted and was reloading. Part of the reloading process involved changing a data type for one of the fields, which made the foreign key from the other table invalid, thus triggering error 1215. I resolved the problem by dropping and then reloading the other table with the new data type for the involved field.

There is a pitfall I have experienced with "Error 1215: Cannot add foreign key constraint" when using Laravel 4, especially with JeffreyWay's Laravel 4 Generators.
In Laravel 4, you can use JeffreyWay's Generators to generate migration files to create tables one-by-one, which means, each migration file generates one table.
You have to be aware of the fact that each migration file is generated with a timestamp in the filename, which gives the files an order. The order of generation is also the order of migration operation when you fire the Artisan CLI command php artisan migrate.
So, if a file asks for a foreign key constraint referring to a key which will be, but not yet, generated in a latter file, the Error 1215 is fired.
In such a case, you have to adjust the order of migration files generation. Generate new files in proper order, copy-in the content, and then delete the disordered old files.

For MySQL (InnoDB) ... get definitions for the columns you want to link:
SELECT * FROM information_schema.columns WHERE
TABLE_NAME IN (tb_name','referenced_table_name') AND
COLUMN_NAME IN ('col_name','referenced_col_name')\G
Compare and verify both column definitions have:
same COLUMN_TYPE(length), same COLATION
It could be necessary to disable/enable the foreign_key mechanism, but be aware if in a production context:
set foreign_key_checks=0;
ALTER TABLE tb_name ADD FOREIGN KEY(col_name) REFERENCES ref_table(ref_column) ON DELETE ...
set foreign_key_checks=1;

I got the same error while trying to add a foreign key. In my case, the problem was caused by the foreign key table's primary key which was marked as unsigned.

Check for table compatibility (engine) with SHOW TABLE STATUS WHERE Name = 'tableName'.
For example, if one table is MyISAM and the other one is InnoDB, you may have this issue.
You can change it thanks to this command:
ALTER TABLE myTable ENGINE = InnoDB;
From documentation.

In my case I had to disable FOREIGN KEY checks as the source tables did not exist.
SET FOREIGN_KEY_CHECKS=0;

I just wanted to add this case as well for VARCHAR foreign key relation. I spent the last week trying to figure this out in MySQL Workbench 8.0 and was finally able to fix the error.
Short Answer:
The character set and collation of the schema, the table, the column, the referencing table, the referencing column and any other tables that reference to the parent table have to match.
Long Answer:
I had an ENUM datatype in my table. I changed this to VARCHAR and I can get the values from a reference table so that I don't have to alter the parent table to add additional options. This foreign-key relationship seemed straightforward but I got 1215 error. arvind's answer and the following link suggested the use of
SHOW ENGINE INNODB STATUS;
On using this command I got the following verbose description for the error with no additional helpful information
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.
Note that the internal storage type of ENUM and SET changed in
tables created with >= InnoDB-4.1.12, and such columns in old tables
cannot be referenced by such columns in new tables.
Please refer to http://dev.mysql.com/doc/refman/8.0/en/innodb-foreign-key-constraints.html for correct foreign key definition.
After which I used SET FOREIGN_KEY_CHECKS=0; as suggested by Arvind Bharadwaj and the link here:
This gave the following error message:
Error Code: 1822. Failed to add the foreign key constraint. Missing
index for constraint
At this point, I 'reverse engineer'-ed the schema and I was able to make the foreign-key relationship in the EER diagram. On 'forward engineer'-ing, I got the following error:
Error 1452: Cannot add or update a child row: a foreign key constraint
fails
When I 'forward engineer'-ed the EER diagram to a new schema, the SQL script ran without issues. On comparing the generated SQL from the attempts to forward engineer, I found that the difference was the character set and collation. The parent table, child table and the two columns had utf8mb4 character set and utf8mb4_0900_ai_ci collation, however, another column in the parent table was referenced using CHARACTER SET = utf8 , COLLATE = utf8_bin ; to a different child table.
For the entire schema, I changed the character set and collation for all the tables and all the columns to the following:
CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci;
This finally solved my problem with 1215 error.
Side Note:
The collation utf8mb4_general_ci works in MySQL Workbench 5.0 or later. Collation utf8mb4_0900_ai_ci works just for MySQL Workbench 8.0 or higher. I believe one of the reasons I had issues with character set and collation is due to MySQL Workbench upgrade to 8.0 in between. Here is a link that talks more about this collation.

I had the same problem.
I solved it doing this:
I created the following line in the
primary key: (id int(11) unsigned NOT NULL AUTO_INCREMENT)
I found out this solution after trying to import a table in my schema builder.

I had the same issue, and my solution is:
Before:
CREATE TABLE EMPRES
( NoFilm smallint NOT NULL
PRIMARY KEY (NoFilm)
FOREIGN KEY (NoFilm) REFERENCES cassettes
);
Solution:
CREATE TABLE EMPRES
(NoFilm smallint NOT NULL REFERENCES cassettes,
PRIMARY KEY (NoFilm)
);

This also happens when the type of the columns is not the same.
E.g., if the column you are referring to is an UNSIGNED INT and the column being referred to is INT then you get this error.

I can not find this error
CREATE TABLE RATING (
Riv_Id INT(5),
Mov_Id INT(10) DEFAULT 0,
Stars INT(5),
Rating_date DATE,
PRIMARY KEY (Riv_Id, Mov_Id),
FOREIGN KEY (Riv_Id) REFERENCES REVIEWER(Reviewer_ID)
ON DELETE SET NULL ON UPDATE CASCADE,
FOREIGN KEY (Mov_Id) REFERENCES MOVIE(Movie_ID)
ON DELETE SET DEFAULT ON UPDATE CASCADE
)

For me it was the column types. BigINT != INT.
But then it still didn't work.
So I checked the engines. Make sure Table1 = InnoDB and Table = InnoDB

Another reason: if you use ON DELETE SET NULL all columns that are used in the foreign key must allow null values. Someone else found this out in this question.
From my understanding it wouldn't be a problem regarding data integrity, but it seems that MySQL just doesn't support this feature (in 5.7).

So I tried all the fixes above and no luck. I may be missing the error in my tables -just could not find the cause and I kept getting error 1215. So I used this fix.
In my local environment in phpMyAdmin, I exported data from the table in question. I selected format CSV. While still in phpMyAdmin with the table selected, I selected "More->Options". Here I scrolled down to "Copy table to (database.table). Select "Structure only". Rename the table something, maybe just add the word "copy" next to the current table name. Click "Go" This will create a new table. Export the new table and import it to the new or other server. I am also using phpMyAdmin here also. Once imported change the name of the table back to its original name. Select the new table, select import. For format select CSV. Uncheck "enable foreign key checks". Select "Go". So far all is working good.
I posted my fix on my blog.

When trying to make a foreign key when using Laravel migration, like this example:
User table
public function up()
{
Schema::create('flights', function (Blueprint $table) {
$table->increments('id');
$table->string('name');
$table->TinyInteger('color_id')->unsigned();
$table->foreign('color_id')->references('id')->on('colors');
$table->timestamps();
});
}
Colors table
public function up()
{
Schema::create('flights', function (Blueprint $table) {
$table->increments('id');
$table->string('color');
$table->timestamps();
});
}
Sometimes properties didn't work:
[PDOException]
SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint
This error happened because the foreign key (type) in [user table] is different from the primary key (type) in the [colors table].
To solve this problem, you should change the primary key in the [colors table]:
$table->tinyIncrements('id');
When you use the primary key, $table->Increments('id');, you should use Integer as a foreign key:
$table->unsignedInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->tinyIncrements('id'); you should use unsignedTinyInteger as a foreign key:
$table->unsignedTinyInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->smallIncrements('id'); you should use unsignedSmallInteger as a foreign key:
$table->unsignedSmallInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->mediumIncrements('id'); you should use unsignedMediumInteger as a foreign key:
$table->unsignedMediumInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');

When this error occurs because the referenced table uses the MyISAM engine, this answer provides a quick way to convert your database so all Django model tables use InnoDB: Converting an existing MyISAM database to InnoDB with Django
It's a Django management command called convert_to_innodb.

Wooo, I just got it! It was a mix of a lot of already-posted answers (InnoDB, unsigned, etc.).
One thing I didn't see here though is: if your foreign key is pointing to a primary key, ensure the source column has a value that makes sense. For example, if the primary key is a mediumint(8), make sure the source column also contains a mediumint(8). That was part of the problem for me.

I experienced this error for a completely different reason. I used MySQL Workbench 6.3 for creating my data model (awesome tool). I noticed that when the column order defined in the foreign key constraint definition does not fit the table column sequence, this error is also generated.
It took me about four hours of trying everything else but checking that.
Now all is working well and I can get back to coding. :-)

This is a subtle version of what has already been said, but in my instance, I had 2 databases (foo and bar). I created foo first and I didn't realize it referenced a foreign key in bar.baz (which wasn't created yet). When I tried to create bar.baz (without any foreign keys), I kept getting this error. After looking around for a while I found the foreign key in foo.
So, long story short, If you get this error, you may have a pre-existing foreign key to the table being created.

As well as all of the previous advice for making sure that fields are identically defined, and table types also have the same collation, make sure that you don't make the rookie mistake of trying to link fields where data in the child field is not already in the parent field. If you have data that is in the child field that you have not already entered in to the parent field then that will cause this error. It's a shame that the error message is not a bit more helpful.
If you are unsure, then back up the table that has the foreign key, delete all the data and then try to create the foreign key. If successful then you know what to do!

Be aware of the use of backticks too. I had in a script the following statement
ALTER TABLE service ADD FOREIGN KEY (create_by) REFERENCES `system_user(id)`;
but the backticks at the end were false. It should have been:
ALTER TABLE service ADD FOREIGN KEY (create_by) REFERENCES `system_user`(`id`);
MySQL unfortunately does not give any details on this error...

Another source of this error is when you have two or more of the same table names which have the same foreign key names.
This sometimes happens to people who use modelling and design software, like MySQL Workbench, and later generate the script from the design.

For me, the 1215 error occurred when I was importing a dumpfile created by mysqldump, which creates the tables alphabetically, which in my case, caused foreign keys to reference tables created later in the file. (Props to this blog post for pointing it out: MySQL Error Code 1215: “Cannot add foreign key constraint”)
Since mysqldump orders tables alphabetically and I did not want to change the names of tables, I followed the instructions in the answer by JeremyWeir on this page, which states to put set FOREIGN_KEY_CHECKS = 0; at the top of the dump file and put SET FOREIGN_KEY_CHECKS = 1; at the bottom of the dump file.
That solution worked for me.

You may also check the Engine of both tables is set to InnoDB.

SQL Cannot add foreign key restraint [duplicate]

I am trying to forward engineer my new schema onto my database server, but I can't figure out why I am getting this error.
I've tried to search for the answer here, but everything I've found has said to either set the database engine to InnoDB or to make sure the keys I'm trying to use as a foreign key are primary keys in their own tables. I have done both of these things, if I'm not mistaken. What else can I do?
Executing SQL script in server
ERROR: Error 1215: Cannot add foreign key constraint
-- -----------------------------------------------------
-- Table `Alternative_Pathways`.`Clients_has_Staff`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Clients_has_Staff` (
`Clients_Case_Number` INT NOT NULL ,
`Staff_Emp_ID` INT NOT NULL ,
PRIMARY KEY (`Clients_Case_Number`, `Staff_Emp_ID`) ,
INDEX `fk_Clients_has_Staff_Staff1_idx` (`Staff_Emp_ID` ASC) ,
INDEX `fk_Clients_has_Staff_Clients_idx` (`Clients_Case_Number` ASC) ,
CONSTRAINT `fk_Clients_has_Staff_Clients`
FOREIGN KEY (`Clients_Case_Number` )
REFERENCES `Alternative_Pathways`.`Clients` (`Case_Number` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_Clients_has_Staff_Staff1`
FOREIGN KEY (`Staff_Emp_ID` )
REFERENCES `Alternative_Pathways`.`Staff` (`Emp_ID` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB
SQL script execution finished: statements: 7 succeeded, 1 failed
Here is the SQL for the parent tables.
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Clients` (
`Case_Number` INT NOT NULL ,
`First_Name` CHAR(10) NULL ,
`Middle_Name` CHAR(10) NULL ,
`Last_Name` CHAR(10) NULL ,
`Address` CHAR(50) NULL ,
`Phone_Number` INT(10) NULL ,
PRIMARY KEY (`Case_Number`) )
ENGINE = InnoDB
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Staff` (
`Emp_ID` INT NOT NULL ,
`First_Name` CHAR(10) NULL ,
`Middle_Name` CHAR(10) NULL ,
`Last_Name` CHAR(10) NULL ,
PRIMARY KEY (`Emp_ID`) )
ENGINE = InnoDB

I'm guessing that Clients.Case_Number and/or Staff.Emp_ID are not exactly the same data type as Clients_has_Staff.Clients_Case_Number and Clients_has_Staff.Staff_Emp_ID.
Perhaps the columns in the parent tables are INT UNSIGNED?
They need to be exactly the same data type in both tables.

Reasons you may get a foreign key constraint error:
You are not using InnoDB as the engine on all tables.
You are trying to reference a nonexistent key on the target table. Make sure it is a key on the other table (it can be a primary or unique key, or just a key)
The types of the columns are not the same (an exception is the column on the referencing table can be nullable even if it is not nullable in the referenced table).
If the primary key or foreign key is a varchar, make sure the collation is the same for both.
One of the reasons may also be that the column you are using for ON DELETE SET NULL is not defined to be null. So make sure that the column is set default null.
Check these.

For others, the same error may not always be due to a column type mismatch. You can find out more information about a MySQL foreign key error by issuing the command
SHOW ENGINE INNODB STATUS;
You may find an error near the top of the printed message. Something like
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.

Error 1215 is an annoying one. Explosion Pill's answer covers the basics. You want to make sure to start from there. However, there are more, much more subtle cases to look out for:
For example, when you try to link up PRIMARY KEYs of different tables, make sure to provide proper ON UPDATE and ON DELETE options. E.g.:
...
PRIMARY KEY (`id`),
FOREIGN KEY (`id`) REFERENCES `t` (`other_id`) ON DELETE SET NULL
....
won't fly, because PRIMARY KEYs (such as id) can't be NULL.
I am sure, there are even more, similarly subtle issues when adding these sort of constraints, which is why when coming across constraint errors, always make sure that the constraints and their implications make sense in your current context. Good luck with your error 1215!

Check the collation of the table. Using SHOW TABLE STATUS, you can check information about the tables, including the collation.
Both tables have to have the same collation.
It's happened to me.

In my case, I had deleted a table using SET FOREIGN_KEY_CHECKS=0, then SET FOREIGN_KEY_CHECKS=1 after. When I went to reload the table, I got error 1215. The problem was there was another table in the database that had a foreign key to the table I had deleted and was reloading. Part of the reloading process involved changing a data type for one of the fields, which made the foreign key from the other table invalid, thus triggering error 1215. I resolved the problem by dropping and then reloading the other table with the new data type for the involved field.

There is a pitfall I have experienced with "Error 1215: Cannot add foreign key constraint" when using Laravel 4, especially with JeffreyWay's Laravel 4 Generators.
In Laravel 4, you can use JeffreyWay's Generators to generate migration files to create tables one-by-one, which means, each migration file generates one table.
You have to be aware of the fact that each migration file is generated with a timestamp in the filename, which gives the files an order. The order of generation is also the order of migration operation when you fire the Artisan CLI command php artisan migrate.
So, if a file asks for a foreign key constraint referring to a key which will be, but not yet, generated in a latter file, the Error 1215 is fired.
In such a case, you have to adjust the order of migration files generation. Generate new files in proper order, copy-in the content, and then delete the disordered old files.

For MySQL (InnoDB) ... get definitions for the columns you want to link:
SELECT * FROM information_schema.columns WHERE
TABLE_NAME IN (tb_name','referenced_table_name') AND
COLUMN_NAME IN ('col_name','referenced_col_name')\G
Compare and verify both column definitions have:
same COLUMN_TYPE(length), same COLATION
It could be necessary to disable/enable the foreign_key mechanism, but be aware if in a production context:
set foreign_key_checks=0;
ALTER TABLE tb_name ADD FOREIGN KEY(col_name) REFERENCES ref_table(ref_column) ON DELETE ...
set foreign_key_checks=1;

I got the same error while trying to add a foreign key. In my case, the problem was caused by the foreign key table's primary key which was marked as unsigned.

Check for table compatibility (engine) with SHOW TABLE STATUS WHERE Name = 'tableName'.
For example, if one table is MyISAM and the other one is InnoDB, you may have this issue.
You can change it thanks to this command:
ALTER TABLE myTable ENGINE = InnoDB;
From documentation.

In my case I had to disable FOREIGN KEY checks as the source tables did not exist.
SET FOREIGN_KEY_CHECKS=0;

I had the same problem.
I solved it doing this:
I created the following line in the
primary key: (id int(11) unsigned NOT NULL AUTO_INCREMENT)
I found out this solution after trying to import a table in my schema builder.

I had the same issue, and my solution is:
Before:
CREATE TABLE EMPRES
( NoFilm smallint NOT NULL
PRIMARY KEY (NoFilm)
FOREIGN KEY (NoFilm) REFERENCES cassettes
);
Solution:
CREATE TABLE EMPRES
(NoFilm smallint NOT NULL REFERENCES cassettes,
PRIMARY KEY (NoFilm)
);

This also happens when the type of the columns is not the same.
E.g., if the column you are referring to is an UNSIGNED INT and the column being referred to is INT then you get this error.

I can not find this error
CREATE TABLE RATING (
Riv_Id INT(5),
Mov_Id INT(10) DEFAULT 0,
Stars INT(5),
Rating_date DATE,
PRIMARY KEY (Riv_Id, Mov_Id),
FOREIGN KEY (Riv_Id) REFERENCES REVIEWER(Reviewer_ID)
ON DELETE SET NULL ON UPDATE CASCADE,
FOREIGN KEY (Mov_Id) REFERENCES MOVIE(Movie_ID)
ON DELETE SET DEFAULT ON UPDATE CASCADE
)

For me it was the column types. BigINT != INT.
But then it still didn't work.
So I checked the engines. Make sure Table1 = InnoDB and Table = InnoDB

Another reason: if you use ON DELETE SET NULL all columns that are used in the foreign key must allow null values. Someone else found this out in this question.
From my understanding it wouldn't be a problem regarding data integrity, but it seems that MySQL just doesn't support this feature (in 5.7).

So I tried all the fixes above and no luck. I may be missing the error in my tables -just could not find the cause and I kept getting error 1215. So I used this fix.
In my local environment in phpMyAdmin, I exported data from the table in question. I selected format CSV. While still in phpMyAdmin with the table selected, I selected "More->Options". Here I scrolled down to "Copy table to (database.table). Select "Structure only". Rename the table something, maybe just add the word "copy" next to the current table name. Click "Go" This will create a new table. Export the new table and import it to the new or other server. I am also using phpMyAdmin here also. Once imported change the name of the table back to its original name. Select the new table, select import. For format select CSV. Uncheck "enable foreign key checks". Select "Go". So far all is working good.
I posted my fix on my blog.

When trying to make a foreign key when using Laravel migration, like this example:
User table
public function up()
{
Schema::create('flights', function (Blueprint $table) {
$table->increments('id');
$table->string('name');
$table->TinyInteger('color_id')->unsigned();
$table->foreign('color_id')->references('id')->on('colors');
$table->timestamps();
});
}
Colors table
public function up()
{
Schema::create('flights', function (Blueprint $table) {
$table->increments('id');
$table->string('color');
$table->timestamps();
});
}
Sometimes properties didn't work:
[PDOException]
SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint
This error happened because the foreign key (type) in [user table] is different from the primary key (type) in the [colors table].
To solve this problem, you should change the primary key in the [colors table]:
$table->tinyIncrements('id');
When you use the primary key, $table->Increments('id');, you should use Integer as a foreign key:
$table->unsignedInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->tinyIncrements('id'); you should use unsignedTinyInteger as a foreign key:
$table->unsignedTinyInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->smallIncrements('id'); you should use unsignedSmallInteger as a foreign key:
$table->unsignedSmallInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->mediumIncrements('id'); you should use unsignedMediumInteger as a foreign key:
$table->unsignedMediumInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');

When this error occurs because the referenced table uses the MyISAM engine, this answer provides a quick way to convert your database so all Django model tables use InnoDB: Converting an existing MyISAM database to InnoDB with Django
It's a Django management command called convert_to_innodb.

Wooo, I just got it! It was a mix of a lot of already-posted answers (InnoDB, unsigned, etc.).
One thing I didn't see here though is: if your foreign key is pointing to a primary key, ensure the source column has a value that makes sense. For example, if the primary key is a mediumint(8), make sure the source column also contains a mediumint(8). That was part of the problem for me.

I experienced this error for a completely different reason. I used MySQL Workbench 6.3 for creating my data model (awesome tool). I noticed that when the column order defined in the foreign key constraint definition does not fit the table column sequence, this error is also generated.
It took me about four hours of trying everything else but checking that.
Now all is working well and I can get back to coding. :-)

This is a subtle version of what has already been said, but in my instance, I had 2 databases (foo and bar). I created foo first and I didn't realize it referenced a foreign key in bar.baz (which wasn't created yet). When I tried to create bar.baz (without any foreign keys), I kept getting this error. After looking around for a while I found the foreign key in foo.
So, long story short, If you get this error, you may have a pre-existing foreign key to the table being created.

As well as all of the previous advice for making sure that fields are identically defined, and table types also have the same collation, make sure that you don't make the rookie mistake of trying to link fields where data in the child field is not already in the parent field. If you have data that is in the child field that you have not already entered in to the parent field then that will cause this error. It's a shame that the error message is not a bit more helpful.
If you are unsure, then back up the table that has the foreign key, delete all the data and then try to create the foreign key. If successful then you know what to do!

Be aware of the use of backticks too. I had in a script the following statement
ALTER TABLE service ADD FOREIGN KEY (create_by) REFERENCES `system_user(id)`;
but the backticks at the end were false. It should have been:
ALTER TABLE service ADD FOREIGN KEY (create_by) REFERENCES `system_user`(`id`);
MySQL unfortunately does not give any details on this error...

Another source of this error is when you have two or more of the same table names which have the same foreign key names.
This sometimes happens to people who use modelling and design software, like MySQL Workbench, and later generate the script from the design.

For me, the 1215 error occurred when I was importing a dumpfile created by mysqldump, which creates the tables alphabetically, which in my case, caused foreign keys to reference tables created later in the file. (Props to this blog post for pointing it out: MySQL Error Code 1215: “Cannot add foreign key constraint”)
Since mysqldump orders tables alphabetically and I did not want to change the names of tables, I followed the instructions in the answer by JeremyWeir on this page, which states to put set FOREIGN_KEY_CHECKS = 0; at the top of the dump file and put SET FOREIGN_KEY_CHECKS = 1; at the bottom of the dump file.
That solution worked for me.

You may also check the Engine of both tables is set to InnoDB.

Foreign Key error in MySql [duplicate]

I am trying to forward engineer my new schema onto my database server, but I can't figure out why I am getting this error.
I've tried to search for the answer here, but everything I've found has said to either set the database engine to InnoDB or to make sure the keys I'm trying to use as a foreign key are primary keys in their own tables. I have done both of these things, if I'm not mistaken. What else can I do?
Executing SQL script in server
ERROR: Error 1215: Cannot add foreign key constraint
-- -----------------------------------------------------
-- Table `Alternative_Pathways`.`Clients_has_Staff`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Clients_has_Staff` (
`Clients_Case_Number` INT NOT NULL ,
`Staff_Emp_ID` INT NOT NULL ,
PRIMARY KEY (`Clients_Case_Number`, `Staff_Emp_ID`) ,
INDEX `fk_Clients_has_Staff_Staff1_idx` (`Staff_Emp_ID` ASC) ,
INDEX `fk_Clients_has_Staff_Clients_idx` (`Clients_Case_Number` ASC) ,
CONSTRAINT `fk_Clients_has_Staff_Clients`
FOREIGN KEY (`Clients_Case_Number` )
REFERENCES `Alternative_Pathways`.`Clients` (`Case_Number` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_Clients_has_Staff_Staff1`
FOREIGN KEY (`Staff_Emp_ID` )
REFERENCES `Alternative_Pathways`.`Staff` (`Emp_ID` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB
SQL script execution finished: statements: 7 succeeded, 1 failed
Here is the SQL for the parent tables.
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Clients` (
`Case_Number` INT NOT NULL ,
`First_Name` CHAR(10) NULL ,
`Middle_Name` CHAR(10) NULL ,
`Last_Name` CHAR(10) NULL ,
`Address` CHAR(50) NULL ,
`Phone_Number` INT(10) NULL ,
PRIMARY KEY (`Case_Number`) )
ENGINE = InnoDB
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Staff` (
`Emp_ID` INT NOT NULL ,
`First_Name` CHAR(10) NULL ,
`Middle_Name` CHAR(10) NULL ,
`Last_Name` CHAR(10) NULL ,
PRIMARY KEY (`Emp_ID`) )
ENGINE = InnoDB

I'm guessing that Clients.Case_Number and/or Staff.Emp_ID are not exactly the same data type as Clients_has_Staff.Clients_Case_Number and Clients_has_Staff.Staff_Emp_ID.
Perhaps the columns in the parent tables are INT UNSIGNED?
They need to be exactly the same data type in both tables.

Reasons you may get a foreign key constraint error:
You are not using InnoDB as the engine on all tables.
You are trying to reference a nonexistent key on the target table. Make sure it is a key on the other table (it can be a primary or unique key, or just a key)
The types of the columns are not the same (an exception is the column on the referencing table can be nullable even if it is not nullable in the referenced table).
If the primary key or foreign key is a varchar, make sure the collation is the same for both.
One of the reasons may also be that the column you are using for ON DELETE SET NULL is not defined to be null. So make sure that the column is set default null.
Check these.

For others, the same error may not always be due to a column type mismatch. You can find out more information about a MySQL foreign key error by issuing the command
SHOW ENGINE INNODB STATUS;
You may find an error near the top of the printed message. Something like
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.

Error 1215 is an annoying one. Explosion Pill's answer covers the basics. You want to make sure to start from there. However, there are more, much more subtle cases to look out for:
For example, when you try to link up PRIMARY KEYs of different tables, make sure to provide proper ON UPDATE and ON DELETE options. E.g.:
...
PRIMARY KEY (`id`),
FOREIGN KEY (`id`) REFERENCES `t` (`other_id`) ON DELETE SET NULL
....
won't fly, because PRIMARY KEYs (such as id) can't be NULL.
I am sure, there are even more, similarly subtle issues when adding these sort of constraints, which is why when coming across constraint errors, always make sure that the constraints and their implications make sense in your current context. Good luck with your error 1215!

Check the collation of the table. Using SHOW TABLE STATUS, you can check information about the tables, including the collation.
Both tables have to have the same collation.
It's happened to me.

In my case, I had deleted a table using SET FOREIGN_KEY_CHECKS=0, then SET FOREIGN_KEY_CHECKS=1 after. When I went to reload the table, I got error 1215. The problem was there was another table in the database that had a foreign key to the table I had deleted and was reloading. Part of the reloading process involved changing a data type for one of the fields, which made the foreign key from the other table invalid, thus triggering error 1215. I resolved the problem by dropping and then reloading the other table with the new data type for the involved field.

There is a pitfall I have experienced with "Error 1215: Cannot add foreign key constraint" when using Laravel 4, especially with JeffreyWay's Laravel 4 Generators.
In Laravel 4, you can use JeffreyWay's Generators to generate migration files to create tables one-by-one, which means, each migration file generates one table.
You have to be aware of the fact that each migration file is generated with a timestamp in the filename, which gives the files an order. The order of generation is also the order of migration operation when you fire the Artisan CLI command php artisan migrate.
So, if a file asks for a foreign key constraint referring to a key which will be, but not yet, generated in a latter file, the Error 1215 is fired.
In such a case, you have to adjust the order of migration files generation. Generate new files in proper order, copy-in the content, and then delete the disordered old files.

For MySQL (InnoDB) ... get definitions for the columns you want to link:
SELECT * FROM information_schema.columns WHERE
TABLE_NAME IN (tb_name','referenced_table_name') AND
COLUMN_NAME IN ('col_name','referenced_col_name')\G
Compare and verify both column definitions have:
same COLUMN_TYPE(length), same COLATION
It could be necessary to disable/enable the foreign_key mechanism, but be aware if in a production context:
set foreign_key_checks=0;
ALTER TABLE tb_name ADD FOREIGN KEY(col_name) REFERENCES ref_table(ref_column) ON DELETE ...
set foreign_key_checks=1;

I got the same error while trying to add a foreign key. In my case, the problem was caused by the foreign key table's primary key which was marked as unsigned.

Check for table compatibility (engine) with SHOW TABLE STATUS WHERE Name = 'tableName'.
For example, if one table is MyISAM and the other one is InnoDB, you may have this issue.
You can change it thanks to this command:
ALTER TABLE myTable ENGINE = InnoDB;
From documentation.

In my case I had to disable FOREIGN KEY checks as the source tables did not exist.
SET FOREIGN_KEY_CHECKS=0;

I had the same problem.
I solved it doing this:
I created the following line in the
primary key: (id int(11) unsigned NOT NULL AUTO_INCREMENT)
I found out this solution after trying to import a table in my schema builder.

I had the same issue, and my solution is:
Before:
CREATE TABLE EMPRES
( NoFilm smallint NOT NULL
PRIMARY KEY (NoFilm)
FOREIGN KEY (NoFilm) REFERENCES cassettes
);
Solution:
CREATE TABLE EMPRES
(NoFilm smallint NOT NULL REFERENCES cassettes,
PRIMARY KEY (NoFilm)
);

This also happens when the type of the columns is not the same.
E.g., if the column you are referring to is an UNSIGNED INT and the column being referred to is INT then you get this error.

I can not find this error
CREATE TABLE RATING (
Riv_Id INT(5),
Mov_Id INT(10) DEFAULT 0,
Stars INT(5),
Rating_date DATE,
PRIMARY KEY (Riv_Id, Mov_Id),
FOREIGN KEY (Riv_Id) REFERENCES REVIEWER(Reviewer_ID)
ON DELETE SET NULL ON UPDATE CASCADE,
FOREIGN KEY (Mov_Id) REFERENCES MOVIE(Movie_ID)
ON DELETE SET DEFAULT ON UPDATE CASCADE
)

For me it was the column types. BigINT != INT.
But then it still didn't work.
So I checked the engines. Make sure Table1 = InnoDB and Table = InnoDB

Another reason: if you use ON DELETE SET NULL all columns that are used in the foreign key must allow null values. Someone else found this out in this question.
From my understanding it wouldn't be a problem regarding data integrity, but it seems that MySQL just doesn't support this feature (in 5.7).

So I tried all the fixes above and no luck. I may be missing the error in my tables -just could not find the cause and I kept getting error 1215. So I used this fix.
In my local environment in phpMyAdmin, I exported data from the table in question. I selected format CSV. While still in phpMyAdmin with the table selected, I selected "More->Options". Here I scrolled down to "Copy table to (database.table). Select "Structure only". Rename the table something, maybe just add the word "copy" next to the current table name. Click "Go" This will create a new table. Export the new table and import it to the new or other server. I am also using phpMyAdmin here also. Once imported change the name of the table back to its original name. Select the new table, select import. For format select CSV. Uncheck "enable foreign key checks". Select "Go". So far all is working good.
I posted my fix on my blog.

When trying to make a foreign key when using Laravel migration, like this example:
User table
public function up()
{
Schema::create('flights', function (Blueprint $table) {
$table->increments('id');
$table->string('name');
$table->TinyInteger('color_id')->unsigned();
$table->foreign('color_id')->references('id')->on('colors');
$table->timestamps();
});
}
Colors table
public function up()
{
Schema::create('flights', function (Blueprint $table) {
$table->increments('id');
$table->string('color');
$table->timestamps();
});
}
Sometimes properties didn't work:
[PDOException]
SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint
This error happened because the foreign key (type) in [user table] is different from the primary key (type) in the [colors table].
To solve this problem, you should change the primary key in the [colors table]:
$table->tinyIncrements('id');
When you use the primary key, $table->Increments('id');, you should use Integer as a foreign key:
$table->unsignedInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->tinyIncrements('id'); you should use unsignedTinyInteger as a foreign key:
$table->unsignedTinyInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->smallIncrements('id'); you should use unsignedSmallInteger as a foreign key:
$table->unsignedSmallInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->mediumIncrements('id'); you should use unsignedMediumInteger as a foreign key:
$table->unsignedMediumInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');

When this error occurs because the referenced table uses the MyISAM engine, this answer provides a quick way to convert your database so all Django model tables use InnoDB: Converting an existing MyISAM database to InnoDB with Django
It's a Django management command called convert_to_innodb.

Wooo, I just got it! It was a mix of a lot of already-posted answers (InnoDB, unsigned, etc.).
One thing I didn't see here though is: if your foreign key is pointing to a primary key, ensure the source column has a value that makes sense. For example, if the primary key is a mediumint(8), make sure the source column also contains a mediumint(8). That was part of the problem for me.

I experienced this error for a completely different reason. I used MySQL Workbench 6.3 for creating my data model (awesome tool). I noticed that when the column order defined in the foreign key constraint definition does not fit the table column sequence, this error is also generated.
It took me about four hours of trying everything else but checking that.
Now all is working well and I can get back to coding. :-)

This is a subtle version of what has already been said, but in my instance, I had 2 databases (foo and bar). I created foo first and I didn't realize it referenced a foreign key in bar.baz (which wasn't created yet). When I tried to create bar.baz (without any foreign keys), I kept getting this error. After looking around for a while I found the foreign key in foo.
So, long story short, If you get this error, you may have a pre-existing foreign key to the table being created.

As well as all of the previous advice for making sure that fields are identically defined, and table types also have the same collation, make sure that you don't make the rookie mistake of trying to link fields where data in the child field is not already in the parent field. If you have data that is in the child field that you have not already entered in to the parent field then that will cause this error. It's a shame that the error message is not a bit more helpful.
If you are unsure, then back up the table that has the foreign key, delete all the data and then try to create the foreign key. If successful then you know what to do!

Be aware of the use of backticks too. I had in a script the following statement
ALTER TABLE service ADD FOREIGN KEY (create_by) REFERENCES `system_user(id)`;
but the backticks at the end were false. It should have been:
ALTER TABLE service ADD FOREIGN KEY (create_by) REFERENCES `system_user`(`id`);
MySQL unfortunately does not give any details on this error...

Another source of this error is when you have two or more of the same table names which have the same foreign key names.
This sometimes happens to people who use modelling and design software, like MySQL Workbench, and later generate the script from the design.

For me, the 1215 error occurred when I was importing a dumpfile created by mysqldump, which creates the tables alphabetically, which in my case, caused foreign keys to reference tables created later in the file. (Props to this blog post for pointing it out: MySQL Error Code 1215: “Cannot add foreign key constraint”)
Since mysqldump orders tables alphabetically and I did not want to change the names of tables, I followed the instructions in the answer by JeremyWeir on this page, which states to put set FOREIGN_KEY_CHECKS = 0; at the top of the dump file and put SET FOREIGN_KEY_CHECKS = 1; at the bottom of the dump file.
That solution worked for me.

You may also check the Engine of both tables is set to InnoDB.

MySQL Error 1215: Cannot add foreign key constraint

I am trying to forward engineer my new schema onto my database server, but I can't figure out why I am getting this error.
I've tried to search for the answer here, but everything I've found has said to either set the database engine to InnoDB or to make sure the keys I'm trying to use as a foreign key are primary keys in their own tables. I have done both of these things, if I'm not mistaken. What else can I do?
Executing SQL script in server
ERROR: Error 1215: Cannot add foreign key constraint
-- -----------------------------------------------------
-- Table `Alternative_Pathways`.`Clients_has_Staff`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Clients_has_Staff` (
`Clients_Case_Number` INT NOT NULL ,
`Staff_Emp_ID` INT NOT NULL ,
PRIMARY KEY (`Clients_Case_Number`, `Staff_Emp_ID`) ,
INDEX `fk_Clients_has_Staff_Staff1_idx` (`Staff_Emp_ID` ASC) ,
INDEX `fk_Clients_has_Staff_Clients_idx` (`Clients_Case_Number` ASC) ,
CONSTRAINT `fk_Clients_has_Staff_Clients`
FOREIGN KEY (`Clients_Case_Number` )
REFERENCES `Alternative_Pathways`.`Clients` (`Case_Number` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_Clients_has_Staff_Staff1`
FOREIGN KEY (`Staff_Emp_ID` )
REFERENCES `Alternative_Pathways`.`Staff` (`Emp_ID` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB
SQL script execution finished: statements: 7 succeeded, 1 failed
Here is the SQL for the parent tables.
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Clients` (
`Case_Number` INT NOT NULL ,
`First_Name` CHAR(10) NULL ,
`Middle_Name` CHAR(10) NULL ,
`Last_Name` CHAR(10) NULL ,
`Address` CHAR(50) NULL ,
`Phone_Number` INT(10) NULL ,
PRIMARY KEY (`Case_Number`) )
ENGINE = InnoDB
CREATE TABLE IF NOT EXISTS `Alternative_Pathways`.`Staff` (
`Emp_ID` INT NOT NULL ,
`First_Name` CHAR(10) NULL ,
`Middle_Name` CHAR(10) NULL ,
`Last_Name` CHAR(10) NULL ,
PRIMARY KEY (`Emp_ID`) )
ENGINE = InnoDB

I'm guessing that Clients.Case_Number and/or Staff.Emp_ID are not exactly the same data type as Clients_has_Staff.Clients_Case_Number and Clients_has_Staff.Staff_Emp_ID.
Perhaps the columns in the parent tables are INT UNSIGNED?
They need to be exactly the same data type in both tables.

Reasons you may get a foreign key constraint error:
You are not using InnoDB as the engine on all tables.
You are trying to reference a nonexistent key on the target table. Make sure it is a key on the other table (it can be a primary or unique key, or just a key)
The types of the columns are not the same (an exception is the column on the referencing table can be nullable even if it is not nullable in the referenced table).
If the primary key or foreign key is a varchar, make sure the collation is the same for both.
One of the reasons may also be that the column you are using for ON DELETE SET NULL is not defined to be null. So make sure that the column is set default null.
Check these.

For others, the same error may not always be due to a column type mismatch. You can find out more information about a MySQL foreign key error by issuing the command
SHOW ENGINE INNODB STATUS;
You may find an error near the top of the printed message. Something like
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.

Error 1215 is an annoying one. Explosion Pill's answer covers the basics. You want to make sure to start from there. However, there are more, much more subtle cases to look out for:
For example, when you try to link up PRIMARY KEYs of different tables, make sure to provide proper ON UPDATE and ON DELETE options. E.g.:
...
PRIMARY KEY (`id`),
FOREIGN KEY (`id`) REFERENCES `t` (`other_id`) ON DELETE SET NULL
....
won't fly, because PRIMARY KEYs (such as id) can't be NULL.
I am sure, there are even more, similarly subtle issues when adding these sort of constraints, which is why when coming across constraint errors, always make sure that the constraints and their implications make sense in your current context. Good luck with your error 1215!

Check the collation of the table. Using SHOW TABLE STATUS, you can check information about the tables, including the collation.
Both tables have to have the same collation.
It's happened to me.

In my case, I had deleted a table using SET FOREIGN_KEY_CHECKS=0, then SET FOREIGN_KEY_CHECKS=1 after. When I went to reload the table, I got error 1215. The problem was there was another table in the database that had a foreign key to the table I had deleted and was reloading. Part of the reloading process involved changing a data type for one of the fields, which made the foreign key from the other table invalid, thus triggering error 1215. I resolved the problem by dropping and then reloading the other table with the new data type for the involved field.

There is a pitfall I have experienced with "Error 1215: Cannot add foreign key constraint" when using Laravel 4, especially with JeffreyWay's Laravel 4 Generators.
In Laravel 4, you can use JeffreyWay's Generators to generate migration files to create tables one-by-one, which means, each migration file generates one table.
You have to be aware of the fact that each migration file is generated with a timestamp in the filename, which gives the files an order. The order of generation is also the order of migration operation when you fire the Artisan CLI command php artisan migrate.
So, if a file asks for a foreign key constraint referring to a key which will be, but not yet, generated in a latter file, the Error 1215 is fired.
In such a case, you have to adjust the order of migration files generation. Generate new files in proper order, copy-in the content, and then delete the disordered old files.

For MySQL (InnoDB) ... get definitions for the columns you want to link:
SELECT * FROM information_schema.columns WHERE
TABLE_NAME IN (tb_name','referenced_table_name') AND
COLUMN_NAME IN ('col_name','referenced_col_name')\G
Compare and verify both column definitions have:
same COLUMN_TYPE(length), same COLATION
It could be necessary to disable/enable the foreign_key mechanism, but be aware if in a production context:
set foreign_key_checks=0;
ALTER TABLE tb_name ADD FOREIGN KEY(col_name) REFERENCES ref_table(ref_column) ON DELETE ...
set foreign_key_checks=1;

I got the same error while trying to add a foreign key. In my case, the problem was caused by the foreign key table's primary key which was marked as unsigned.

Check for table compatibility (engine) with SHOW TABLE STATUS WHERE Name = 'tableName'.
For example, if one table is MyISAM and the other one is InnoDB, you may have this issue.
You can change it thanks to this command:
ALTER TABLE myTable ENGINE = InnoDB;
From documentation.

In my case I had to disable FOREIGN KEY checks as the source tables did not exist.
SET FOREIGN_KEY_CHECKS=0;

I had the same problem.
I solved it doing this:
I created the following line in the
primary key: (id int(11) unsigned NOT NULL AUTO_INCREMENT)
I found out this solution after trying to import a table in my schema builder.

I had the same issue, and my solution is:
Before:
CREATE TABLE EMPRES
( NoFilm smallint NOT NULL
PRIMARY KEY (NoFilm)
FOREIGN KEY (NoFilm) REFERENCES cassettes
);
Solution:
CREATE TABLE EMPRES
(NoFilm smallint NOT NULL REFERENCES cassettes,
PRIMARY KEY (NoFilm)
);

This also happens when the type of the columns is not the same.
E.g., if the column you are referring to is an UNSIGNED INT and the column being referred to is INT then you get this error.

I can not find this error
CREATE TABLE RATING (
Riv_Id INT(5),
Mov_Id INT(10) DEFAULT 0,
Stars INT(5),
Rating_date DATE,
PRIMARY KEY (Riv_Id, Mov_Id),
FOREIGN KEY (Riv_Id) REFERENCES REVIEWER(Reviewer_ID)
ON DELETE SET NULL ON UPDATE CASCADE,
FOREIGN KEY (Mov_Id) REFERENCES MOVIE(Movie_ID)
ON DELETE SET DEFAULT ON UPDATE CASCADE
)

For me it was the column types. BigINT != INT.
But then it still didn't work.
So I checked the engines. Make sure Table1 = InnoDB and Table = InnoDB

Another reason: if you use ON DELETE SET NULL all columns that are used in the foreign key must allow null values. Someone else found this out in this question.
From my understanding it wouldn't be a problem regarding data integrity, but it seems that MySQL just doesn't support this feature (in 5.7).

So I tried all the fixes above and no luck. I may be missing the error in my tables -just could not find the cause and I kept getting error 1215. So I used this fix.
In my local environment in phpMyAdmin, I exported data from the table in question. I selected format CSV. While still in phpMyAdmin with the table selected, I selected "More->Options". Here I scrolled down to "Copy table to (database.table). Select "Structure only". Rename the table something, maybe just add the word "copy" next to the current table name. Click "Go" This will create a new table. Export the new table and import it to the new or other server. I am also using phpMyAdmin here also. Once imported change the name of the table back to its original name. Select the new table, select import. For format select CSV. Uncheck "enable foreign key checks". Select "Go". So far all is working good.
I posted my fix on my blog.

When trying to make a foreign key when using Laravel migration, like this example:
User table
public function up()
{
Schema::create('flights', function (Blueprint $table) {
$table->increments('id');
$table->string('name');
$table->TinyInteger('color_id')->unsigned();
$table->foreign('color_id')->references('id')->on('colors');
$table->timestamps();
});
}
Colors table
public function up()
{
Schema::create('flights', function (Blueprint $table) {
$table->increments('id');
$table->string('color');
$table->timestamps();
});
}
Sometimes properties didn't work:
[PDOException]
SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint
This error happened because the foreign key (type) in [user table] is different from the primary key (type) in the [colors table].
To solve this problem, you should change the primary key in the [colors table]:
$table->tinyIncrements('id');
When you use the primary key, $table->Increments('id');, you should use Integer as a foreign key:
$table->unsignedInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->tinyIncrements('id'); you should use unsignedTinyInteger as a foreign key:
$table->unsignedTinyInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->smallIncrements('id'); you should use unsignedSmallInteger as a foreign key:
$table->unsignedSmallInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');
When you use primary key $table->mediumIncrements('id'); you should use unsignedMediumInteger as a foreign key:
$table->unsignedMediumInteger('fk_id');
$table->foreign('fk_id')->references('id')->on('table_name');

When this error occurs because the referenced table uses the MyISAM engine, this answer provides a quick way to convert your database so all Django model tables use InnoDB: Converting an existing MyISAM database to InnoDB with Django
It's a Django management command called convert_to_innodb.

Wooo, I just got it! It was a mix of a lot of already-posted answers (InnoDB, unsigned, etc.).
One thing I didn't see here though is: if your foreign key is pointing to a primary key, ensure the source column has a value that makes sense. For example, if the primary key is a mediumint(8), make sure the source column also contains a mediumint(8). That was part of the problem for me.

I experienced this error for a completely different reason. I used MySQL Workbench 6.3 for creating my data model (awesome tool). I noticed that when the column order defined in the foreign key constraint definition does not fit the table column sequence, this error is also generated.
It took me about four hours of trying everything else but checking that.
Now all is working well and I can get back to coding. :-)

This is a subtle version of what has already been said, but in my instance, I had 2 databases (foo and bar). I created foo first and I didn't realize it referenced a foreign key in bar.baz (which wasn't created yet). When I tried to create bar.baz (without any foreign keys), I kept getting this error. After looking around for a while I found the foreign key in foo.
So, long story short, If you get this error, you may have a pre-existing foreign key to the table being created.

As well as all of the previous advice for making sure that fields are identically defined, and table types also have the same collation, make sure that you don't make the rookie mistake of trying to link fields where data in the child field is not already in the parent field. If you have data that is in the child field that you have not already entered in to the parent field then that will cause this error. It's a shame that the error message is not a bit more helpful.
If you are unsure, then back up the table that has the foreign key, delete all the data and then try to create the foreign key. If successful then you know what to do!

Be aware of the use of backticks too. I had in a script the following statement
ALTER TABLE service ADD FOREIGN KEY (create_by) REFERENCES `system_user(id)`;
but the backticks at the end were false. It should have been:
ALTER TABLE service ADD FOREIGN KEY (create_by) REFERENCES `system_user`(`id`);
MySQL unfortunately does not give any details on this error...

Another source of this error is when you have two or more of the same table names which have the same foreign key names.
This sometimes happens to people who use modelling and design software, like MySQL Workbench, and later generate the script from the design.

For me, the 1215 error occurred when I was importing a dumpfile created by mysqldump, which creates the tables alphabetically, which in my case, caused foreign keys to reference tables created later in the file. (Props to this blog post for pointing it out: MySQL Error Code 1215: “Cannot add foreign key constraint”)
Since mysqldump orders tables alphabetically and I did not want to change the names of tables, I followed the instructions in the answer by JeremyWeir on this page, which states to put set FOREIGN_KEY_CHECKS = 0; at the top of the dump file and put SET FOREIGN_KEY_CHECKS = 1; at the bottom of the dump file.
That solution worked for me.

You may also check the Engine of both tables is set to InnoDB.

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