I have a foreign key that references two columns in a child table. Id like to delete the foreign key but mySQL just hangs and nothing happens:
Here is the output from SHOW CREATE TABLE table_name:
KEY FK_animal_index (animal_type,food_index),
CONSTRAINT FK_animal_index FOREIGN KEY (animal_type, food_index)
REFERENCES animal_schedules (animal_type, food_index)
ENGINE=InnoDB
I have tried deleting this foreign key using:
`ALTER TABLE table_name DROP FOREIGN KEY FK_animal_index;`
`ALTER TABLE table_name DROP FOREIGN KEY animal_type;`
ALTER TABLE section_configuration DROP FOREIGN KEY FK_animal_index,
DROP KEY (animal_type, food_index);
AND
ALTER TABLE section_configuration DROP FOREIGN KEYFK_animal_index, ADD CONSTRAINT FK_animal_type FOREIGN KEY (animal_type) REFERENCESanimal_schedules(animal_type) ON DELETE SET NULL;
Others have mentioned that ON DELETE not being set can prevent you changing key values (mySQL will reject the change if ON DELETE is not set to anything)
But to no avail, mySQL just hangs and 30 minutes later still nothing. The database is very small at the moment so removing the FK should be fast.
The answer to this question was not about primary keys at all.
I had to stop my program from accessing the table in order to make modifications to it.
In mySQL db, you can add tables and add columns at any time, but if you want to change a column or remove a column, it must not be in use by any program or it will hang indefinitely.
On PHPMyAdmin, I would like to DROP a FOREIGN KEY with
ALTER TABLE information DROP FOREIGN KEY IDX_29791883B30676A7
Because when I do the next query it's not working:
ALTER TABLE information DROP INDEX IDX_29791883B30676A7
Cannot drop index 'IDX_29791883B30676A7': needed in a foreign key constraint
However, the second query as the error that the index is use as foreigner key.
Fine, but when I do the first query I get this error:
Can't DROP 'IDX_29791883B30676A7'; check that column/key exists
So the questions are:
How do I acually check that,
How can I finally drop that key.
Foreign keys by default are prefixed with 'FK' not 'IDX'. You are trying to remove an index instead of a foreign key. You mentioned that the foreign key is: FK_29791883B30676A7 so the correct way to remove it will be:
ALTER TABLE information DROP FOREIGN KEY FK_29791883B30676A7
Once a foreign key has been created, you may find that you wish to drop the foreign key from the table. You can do this with the ALTER TABLE statement in SQL Server
I have the same problem with foreign keys, well I found a solution to removing constraint. For first take a look at:
SHOW CREATE TABLE information;
You will found the name of the rule, and then just drop it:
ALTER TABLE information DROP CONSTRAINT `name_of_rule`;
Try this
SET FOREIGN_KEY_CHECKS=0;
ALTER TABLE information DROP FOREIGN KEY IDX_29791883B30676A7;
ALTER TABLE information DROP INDEX IDX_29791883B30676A7;
SET FOREIGN_KEY_CHECKS=1;
I'm in the midst of modifying my current DB schema. I'd like to drop the child side of a one-to-one relationship, but I keep getting SQL errors about the foreign key restraint, specifically:
#1217 - Cannot delete or update a parent row: a foreign key constraint fails
Even though there doesn't seem to be any foreign key in play at the moment (the child table is completely empty).
Any ideas?
First, DROP the foreign key constraint from the referencing (child) table, and then DROP the referenced (parent) table.
For example:
ALTER TABLE child
DROP FOREIGN KEY FK_child_parent ;
DROP TABLE parent ;
(Obviously, you'd need to replace the "child", "parent" and "FK_child_parent" with the actual identifiers for your tables and foreign key constraint.
One easy way to get the name of the foreign key constraint is to use the SHOW CREATE TABLE statement,
SHOW CREATE TABLE child ;
The output from that will show the name of the foreign key constraint.
If you don't know which tables have a foreign key referencing the table, you can query the information_schema.referential_constraints table to find them
WHERE referenced_table_name = 'parent'
AND constraint_schema = 'mydatabase'
Have you tried
SET foreign_key_checks = 0;
//Do your drop statement
SET foreign_key_checks = 1;
?
In MySQL I want to drop a table.
I tried a lot things but I keep getting the error that the table named bericht can't be dropped. This is the error I'm getting:
#1217 - Cannot delete or update a parent row: a foreign key constraint fails
How do I drop this table?
This should do the trick:
SET FOREIGN_KEY_CHECKS=0; DROP TABLE bericht; SET FOREIGN_KEY_CHECKS=1;
As others point out, this is almost never what you want, even though it's whats asked in the question. A more safe solution is to delete the tables depending on bericht before deleting bericht. See CloudyMarble answer on how to do that. I use bash and the method in my post to drop all tables in a database when I don't want to or can't delete and recreate the database itself.
The #1217 error happens when other tables has foreign key constraints to the table you are trying to delete and you are using the InnoDB database engine. This solution temporarily disables checking the restraints and then re-enables them. Read the documentation for more. Be sure to delete foreign key restraints and fields in tables depending on bericht, otherwise you might leave your database in a broken state.
Try this:
SELECT *
FROM information_schema.KEY_COLUMN_USAGE
WHERE REFERENCED_TABLE_NAME = 'YourTable';
This should deliver you which Tables have references to the table you want to drop, once you drop these references, or the datasets which reference datasets in this table you will be able to drop the table
But fortunately, with the MySQL FOREIGN_KEY_CHECKS variable, you don't have to worry about the order of your DROP TABLE statements at all, and you can write them in any order you like -- even the exact opposite -- like this:
SET FOREIGN_KEY_CHECKS = 0;
drop table if exists customers;
drop table if exists orders;
drop table if exists order_details;
SET FOREIGN_KEY_CHECKS = 1;
For more clarification, check out the link below:
http://alvinalexander.com/blog/post/mysql/drop-mysql-tables-in-any-order-foreign-keys/
Use show create table tbl_name to view the foreign keys
You can use this syntax to drop a foreign key:
ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol
There's also more information here (see Frank Vanderhallen post):
http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html
This probably has the same table to other schema the reason why you're getting that error.
You need to drop first the child row then the parent row.
I realize this is stale for a while and an answer had been selected, but how about the alternative to allow the foreign key to be NULL and then choose ON DELETE SET NULL.
Basically, your table should be changed like so:
ALTER TABLE 'bericht'
DROP FOREIGN KEY 'your_foreign_key';
ALTER TABLE 'bericht'
ADD CONSTRAINT 'your_foreign_key' FOREIGN KEY ('column_foreign_key') REFERENCES 'other_table' ('column_parent_key') ON UPDATE CASCADE ON DELETE SET NULL;
Personally I would recommend using both "ON UPDATE CASCADE" as well as "ON DELETE SET NULL" to avoid unnecessary complications, however your set up may dictate a different approach.
Hope this helps.
I'm setting up a database using phpMyAdmin. I have two tables (foo and bar), indexed on their primary keys. I am trying to create a relational table (foo_bar) between them, using their primary keys as foreign keys.
I created these tables as MyISAM, but have since changed all three to InnoDB, because I read that MyISAM doesn't support foreign keys. All id fields are INT(11).
When I choose the foo_bar table, click the "relation view" link, and try to set the FK columns to be database.foo.id and database.bar.id, it says "No index defined!" beside each column.
What am I missing?
Clarification/Update
For the sake of simplicity, I want to keep using phpMyAdmin. I am currently using XAMPP, which is easy enough to let me focus on the PHP/CSS/Javascript, and it comes with phpMyAdmin.
Also, although I haven't been able to set up explicit foreign keys yet, I do have a relational table and can perform joins like this:
SELECT *
FROM foo
INNER JOIN foo_bar
ON foo.id = foo_bar.foo_id
INNER JOIN bar
ON foo_bar.bar_id = bar.id;
It just makes me uncomfortable not to have the FKs explicitly defined in the database.
If you want to use phpMyAdmin to set up relations, you have to do 2 things. First of all, you have to define an index on the foreign key column in the referring table (so foo_bar.foo_id, in your case). Then, go to relation view (in the referring table) and select the referred column (so in your case foo.id) and the on update and on delete actions.
I think foreign keys are useful if you have multiple tables linked to one another, in particular, your delete scripts will become very short if you set the referencing options correctly.
EDIT: Make sure both of the tables have the InnoDB engine selected.
phpMyAdmin lets you define foreign keys using their "relations" view. But since, MySQL only supports foreign constraints on "INNO DB" tables, the first step is to make sure the tables you are using are of that type.
To setup a foreign key so that the PID column in a table named CHILD references the ID column in a table named PARENT, you can do the following:
For both tables, go to the operations tab and change their type to "INNO DB"
Make sure ID is the primary key (or at least an indexed column) of the PARENT table.
In the CHILD table, define an index for the PID column.
While viewing the structure tab of the CHILD table, click the "relation view" link just above the "add fields" section.
You will be given a table where each row corresponds to an indexed column in your CLIENT table. The first dropdown in each row lets you choose which TABLE->COLUMN the indexed column references. In the row for PID, choose PARENT->ID from the dropdown and click GO.
By doing an export on the CHILD table, you should see a foreign key constraint has been created for the PID column.
This is a summary of a Wikipedia article. It specifies the different types of relationships you can stipulate in PHPmyadmin. I am putting it here because it is relevant to #Nathan's comment on setting the foreign keys options for "on update/delete" but is too large for a comment.
CASCADE
Whenever rows in the master (referenced) table are deleted (resp. updated), the respective rows of the child (referencing) table with a matching foreign key column will get deleted (resp. updated) as well. This is called a cascade delete (resp. update[2]).
RESTRICT
A value cannot be updated or deleted when a row exists in a foreign key table that references the value in the referenced table. Similarly, a row cannot be deleted as long as there is a reference to it from a foreign key table.
NO ACTION
NO ACTION and RESTRICT are very much alike. The main difference between NO ACTION and RESTRICT is that with NO ACTION the referential integrity check is done after trying to alter the table. RESTRICT does the check before trying to execute the UPDATE or DELETE statement. Both referential actions act the same if the referential integrity check fails: the UPDATE or DELETE statement will result in an error.
SET NULL
The foreign key values in the referencing row are set to NULL when the referenced row is updated or deleted. This is only possible if the respective columns in the referencing table are nullable. Due to the semantics of NULL, a referencing row with NULLs in the foreign key columns does not require a referenced row.
SET DEFAULT
Similar to SET NULL, the foreign key values in the referencing row are set to the column default when the referenced row is updated or deleted.
In phpmyadmin, you can assign Foreign key simply by its GUI. Click on the table and go to Structure tab. find the Relation View on just bellow of table (shown in below image).
You can assign the forging key from the list box near by the primary key.(See image below). and save
corresponding SQL query automatically generated and executed.
For those new to database .... and need to ALTER an existing table. A lot things seem to be pretty straightforward, but there is always something ... between A and B.
Before anything else, take a look at this.
Make sure you have P_ID (parent ID on both parent and child table).
Of course it will be already filled in the parent. Not necessarily in the child in a true and final way. So for instance P_ID #3 (maybe many times in the child table will be pointing to original P_ID at parent table).
Go to SQL tab (I am using phpMyAdmin, should be similar in other ones) and do this command:
ALTER TABLE child_table_name
ADD FOREIGN KEY (P_ID)
REFERENCES parent_table_name (P_ID)
Click on child table, than structure, finally on relational view. Finish your DB planning there. There was a nice answer before this one about cascade, restrict, etc.
Of course it could be done by commands...
Foreign key means a non prime attribute of a table referes the prime attribute of another
*in phpMyAdmin* first set the column you want to set foreign key as an index
then click on RELATION VIEW
there u can find the options to set foreign key
InnoDB allows you to add a new foreign key constraint to a table by using ALTER TABLE:
ALTER TABLE tbl_name
ADD [CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (index_col_name, ...)
REFERENCES tbl_name (index_col_name,...)
[ON DELETE reference_option]
[ON UPDATE reference_option]
On the other hand, if MyISAM has advantages over InnoDB in your context, why would you want to create foreign key constraints at all. You can handle this on the model level of your application. Just make sure the columns which you want to use as foreign keys are indexed!
Don't forget that the two columns should have the same data type.
for example if one column is of type INT and the other is of type tinyint you'll get the following error:
Error creating foreign key on [PID column] (check data types)
This is old thread but answer because if useful to anyone.
Step 1. Your Db Storage Engine set to InnoDB
Step 2. Create Primary Table
here customer is primary table and customer_id is primary key
Step 3. create foreign key table and give index
here we have customer_addresses as related table and store customer addresses, so here customer_id relation with customer table
we can select index directly when create table as below
If you forgot to give index when create a table, then you can give index from the structure tab of table as below.
Step 4. Once index give to the field, Go to structure tab and click on Relation View as shown in below pic
Step 5. Now select the ON DELETE and ON UPDATE what you want to do, Select column from current table, select DB (SAME DB), select relation table and primary key from that table as shown in below pic and Save it
Now check if relation are give successfully, go to foreign table data list and click on foreign key value, you will redirect to primary table record, then relation made successfully.
Make sure you have selected your mysql storage engine as Innodb and not MYISAM as Innodb storage engine supports foreign keys in Mysql.
Steps to create foreign keys in phpmyadmin:
Tap on structure for the table which will have the foreign key.
Create INDEX for the column you want to use as foreign key.
Tap on Relation view, placed below the table structure
In the Relation view page, you can see select options in front of the field (which was made an INDEX).
UPDATE CASCADE specifies that the column will be updated when the referenced column is updated,
DELETE CASCADE specified rows will be deleted when the referenced rows are deleted.
Alternatively, you can also trigger sql query for the same
ALTER TABLE table_name
ADD CONSTRAINT fk_foreign_key_name
FOREIGN KEY (foreign_key_name)
REFERENCES target_table(target_key_name);
Step 1:
You have to add the line:
default-storage-engine = InnoDB
under the [mysqld] section of your mysql config file (my.cnf or my.ini depending on your OS) and restart the mysqld service.
Step 2:
Now when you create the table you will see the type of table is: InnoDB
Step 3:
Create both Parent and Child table. Now open the Child table and select the column U like to have the Foreign Key:
Select the Index Key from Action Label as shown below.
Step 4:
Now open the Relation View in the same child table from bottom near the Print View as shown below.
Step 5:
Select the column U like to have the Foreign key as Select the Parent column from the drop down.
dbName.TableName.ColumnName
Select appropriate Values for ON DELETE and ON UPDATE
First set Storage Engine as InnoDB
then the relation view option enable in structure menu
You can also do it with a SQL command, like so.
ALTER TABLE employees
ADD CONSTRAINT fk_companyid FOREIGN KEY (companyid)
REFERENCES companies (id)
ON DELETE CASCADE;
In this example, if a row from companies is deleted, all employees with that companyid are also deleted.
From the official MySQL documentation at https://dev.mysql.com/doc/refman/8.0/en/create-table-foreign-keys.html:
MySQL requires indexes on foreign keys and referenced keys so that
foreign key checks can be fast and not require a table scan.