I'm trying to create a table in mysql with a foreign key reference, like this:
In database A:
CREATE TABLE replication (
id varchar(255) NOT NULL PRIMARY KEY,
uid varchar(255) NOT NULL,
value int(11) NOT NULL,
FOREIGN KEY (uid) REFERENCES databaseB.users(username)
);
In database B i have a table named users like this:
+-----------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+--------------+------+-----+---------+-------+
| id | varchar(255) | NO | | NULL | |
| username | varchar(255) | NO | PRI | NULL | |
+-----------------+--------------+------+-----+---------+-------+
When i try to create table replication, gives me the following error:
ERROR 1005 (HY000): Can't create table 'databaseA.replication' (errno: 150)
Any idea? Thanks in advance!
You need to add an index to uid or you will not be able to set it up to reference anything.
Also, typically in your DatabaseB you would have your users table have a Primary Key on ID and a unique index on username. Then you would set up a foreign key from DatabaseA.replication REFERENCES databaseB.users(id)
you cant make reference key with two different types
in databaseA is integer uid and databaseB is varchar
try this
FOREIGN KEY (uid) REFERENCES databaseB.users(id)
Are both databases of the same type (MyISAM, INNODB)? You cannot point a foreign key to a MyISAM DB.
This error can also occur when you are mixing types for the field type (which is the same in this case, varchar(255)), or when the encodings of the fields are different.
Another reason could be that you don't have access to the other database.
Thetype of the field in a foreign key must be the same as the type of the column they're referencing.I think this problem error because type of the field in a foreign key is different from the column they're referencing.
Your foreign key field is uid and referencing table your field is username.Here is the problem.Try to use same type of field in both table like
FOREIGN KEY (uid) REFERENCES databaseB.users(id)
Hope you understand and works for you.
Related
I have two mysql tables. The first one is created using the following code:
create table project(
project_id int not null auto_increment,
project_name varchar(30)
primary key(project_id));
The second table:
create table matches(
match_id int not null auto_increment,
match_name varchar(30),
project_id int(4) foreign key (project_id) references projects(project_id));
Both these commands works fine. I want to add the project_name column from the first table to the second table. I tried using
alter table projects drop primary key, add primary key(project_id, project_name);
and then
alter table matches add column project_name varchar(30), add foreign key (project_name) references projects(project_name);
But got the following error:
ERROR 1005 (HY000): Can't create table 'matches.#sql-55e_311' (errno: 150)
How do i include both the columns from the first table into the second table.
The current structure of my second table is as follows:
+------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+----------------+
| match_id | int(11) | NO | PRI | NULL | auto_increment |
| match_name | varchar(30) | YES | | NULL | |
| project_id | int(4) | NO | MUL | NULL | |
+------------+-------------+------+-----+---------+----------------+
I want to add the project_name as the fourth column in my second table.
To use a compound Primary Key as Foreign Key, you'll have to add the
same number of columns (that compose the PK) with same datatypes to
the child table and then use the combination of these columns in the
FOREIGN KEY definition.
see related post here https://stackoverflow.com/a/10566463/4904726
so try this way
alter table matches add foreign key (project_id, project_name) references projects(project_id, project_name);
Do you understand what a FK constraint says? It says a list of values for some columns in a table must appear as a list of values for some columns forming a (declared) PK or UNIQUE NOT NULL in the referenced table. So what you are writing doesn't make sense. If you wanted a PK (project_id, project_name) then that should also be the form of your FK.
But (project_id, project_name) is not two 1-column PKs, it is one 2-column PK, and it is probably not what you want since presumably in projects it isn't just pairs that are unique it is each column. Presumably you want two 1-column PKs and two one-column FKs, one referencing each PK.
If projects project_id was NOT NULL you could write:
alter table projects add primary key(project_name);
alter table matches add column project_name varchar(30),
add foreign key (project_name) references projects(project_name);
But if project_name can be NULL in projects then you cannot make it a PK and you cannot sensibly have a FK to it. You can make it UNIQUE. Unfortunately MySQL lets you write such a FK declaration to a non-NULL UNIQUE column while it also tells you not to do it:
The handling of foreign key references to non-unique keys or keys that contain NULL values is not well defined for operations such as UPDATE or DELETE CASCADE. You are advised to use foreign keys that reference only keys that are both UNIQUE (or PRIMARY) and NOT NULL.
So if you want projects project_name to be nullable then should declare it UNIQUE but you should enforce the logical matches project_name match with a trigger.
If I define a column as a primary key in MySQL, is it also unique key by default or do I need to also define it as unique key (in case I want it to be unique)?
I saw this question What is the difference b/w Primary Key and Unique Key that explain the difference between the two, but doesn't exactly answer my question.
Does PK is UK by default or I need to explicitly define it?
Primary key is always unique in every SQL. You dont have to explicitly define it as UNIQUE.
On a side note: You can only have onePrimary key in a table and it never allows null values. Also you can have only one primary key constraint in the table(as the point of creating a primary key is to uniquely identify the row in your table) but you can more than one unique key constraint in your table.
Example:
An employee details table having EmpID as Primary key and EmpPhoneNo as unique key.
Primary key is always unique by definition. Not only in MySQL.
So you don't need any additional unique key.
Note that composite keys may lead to confusion : indeed a primary key can be a composite key, and DESCRIBE will show all of the composite key components as primary keys :
> DESCRIBE foobar;
+----------------------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------------+------------------+------+-----+---------+-------+
| column_A | int(10) unsigned | NO | PRI | NULL | |
| column_B | int(10) unsigned | NO | PRI | NULL | |
+----------------------+------------------+------+-----+---------+-------+
However SHOW CREATE TABLE will show the reality :
> SHOW CREATE TABLE foobar;
+--------+---------------------------…+
| Table | Create Table …|
+--------+---------------------------…+
| foobar | CREATE TABLE `foobar` (
`column_A` int(10) unsigned NOT NULL,
`column_B` int(10) unsigned NOT NULL,
PRIMARY KEY (`column_A`,`column_B`),
KEY `column_B` (`column_B`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+--------+---------------------------…+
I am trying to increase the constraint of my MySQL database schema adding foreign key constraint to each table.
Table 1: USERS
+---------+----------+-------------
| id | username | Other fields
+---------+----------+-------------
| 1 | John |
| 2 | Mark |
+---------+----------+-------------
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT
username` VARCHAR(50) NOT NULL
PRIMARY KEY (id)
Table 2: DISKS (This has a one to many relationship with USERS)
+---------+----------+-----------+-------------
| id | id_user | disk_name | Other fields
+---------+----------+-----------+-------------
| 1 | 1 | disk A |
| 2 | 2 | disk B |
+---------+----------+-----------+-------------
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT
id_user INT(11) NOT NULL,
PRIMARY KEY (id,id_user)
INDEX fk_disks_idx (id ASC)
CONSTRAINT fk_disks
FOREIGN KEY (id)
REFERENCES database.USERS (id)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
Table 3: FILES (This has a one to many relationship with DISKS)
+---------+----------+----------+-----------+-------------
| id | id_disk | id_user | file_name | Other fields
+---------+----------+----------+-----------+-------------
| 1 | 1 | 1 | |
| 2 | 2 | 2 | |
+---------+----------+----------+-----------+-------------
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT
id_user INT(11) NOT NULL
id_disk INT(11) NOT NULL
PRIMARY KEY (id,id_disk,id_user )
INDEX fk_files_idx (id ASC, id_user ASC)
CONSTRAINT fk_files
FOREIGN KEY (id_disk, id_user, id_user)
REFERENCES database.DISKS (id)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
Table 2: FILES_ON_NAS (This has a one to one relationship with FILES)
+-------+----------+----------+----------+-----------+-------------
| id | id_files | id_user | id_disk | file_name | Other fields
+-------+----------+----------+----------+-----------+-------------
| 1 | 1 | 1 | 1 | |
| 2 | 1 | 2 | 2 | |
+-------+----------+----------+----------+-----------+-------------
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT
id_files INT(11) NOT NULL,
id_user INT(11) NOT NULL,
id_disk INT(11) NOT NULL,
PRIMARY KEY (id,id_files,id_user,id_disk )
INDEX fk_files_on_nas_idx (id ASC)
CONSTRAINT fk_files_on_nas
FOREIGN KEY (id_files,id_user,id_disk)
REFERENCES database.FILES (id,id_user, id_disk)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
Question:
As you can see the more I reference table in cascade the more primary keys I get. How can I design the database to avoid the replication of primary keys and therefore data duplication as well? Should I delete the auto incremented key for each table? Is it a good practice?
Thanks
The ID of the disk is sufficient to uniquely identify a disk. So there's no reason to include the ID of the user into the disk's primary key. It would even be an extremely bad idea, because that means that if a disk's user changes, you would need to modify the primary key.
Same for a file. A file ID uniquely identifies a file. So there's no reason to add the disk ID into the primary key of a file.
I would strongly advise against deleting the auto-incremented keys.
However you don't need to make a new primary key everytime :
If you want that multiple users share one disk, just put a foreign key of id_disk in USERS
If you want that one user can have multiple disks, then put a foreign key of id_user in DISKS instead.
Only use primary keys like that when you face a Many-to-Many relationship. In this case, you need to create a new table to join both tables; with the primary keys of both tables as foreign keys making a coupled primary key like you did.
You might want to read a little about Database Normalization. In your case, i would make the surrogate key id the only primary key in the tables. Something like:
create table users (
id integer not null auto_increment,
username varchar(50),
...,
primary key (id)
);
create table disks (
id integer not null auto_increment,
user_id integer,
diskname varchar(50),
....,
primary key (id),
foreign key (user_id) references users (id)
);
For files you are going to have to answer the question: does file ownership depend on the file directly, or transitively on the disk ownership, or are the ownerships independent? A file owned by John on a disk owned by Jack? Seems ok to me, but your domain might have different rules. In that case, drop the user_id from the files table (otherwise your database won't be in Third normal form).
create table files (
id integer not null auto_increment,
disk_id integer,
user_id integer, -- you have to decide whether this is necessary
filename varchar(50),
....,
primary key (id),
foreign key (disk_id) references disks (id),
foreign key (user_id) references users (id)
);
I've run this query on mysql:
ALTER TABLE `connections`
ADD CONSTRAINT `connections_serial_fk`
FOREIGN KEY (`serial`)
REFERENCES `devices`(serial)
ON DELETE CASCADE;
And I'm getting error number 150. I already looked for the documentation and the thing I think that may be causing this error is that the column 'serial' is a varchar(255).
This reference says "innodb permits a foreign key to reference any index column or group of columns".
Does MySql have this restriction?
From my schema:
devices:
| Field | Type | Null | Key | Default | Extra |
| serial | varchar(255) | YES | | NULL | |
connections:
| Field | Type | Null | Key | Default | Extra |
| serial | varchar(255) | YES | | NULL | |
A dbms that conforms to standard SQL will prevent you from setting a foreign key reference to a column (or columns) that doesn't have some kind of unique constraint on it--either PRIMARY KEY or UNIQUE. But MySQL does permit this kind of nonsense under certain circumstances.
Additionally, MySQL requires that the referenced columns be indexed
for performance reasons. However, the system does not enforce a
requirement that the referenced columns be UNIQUE or be declared NOT
NULL. The handling of foreign key references to nonunique keys or keys
that contain NULL values is not well defined for operations such as
UPDATE or DELETE CASCADE. You are advised to use foreign keys that
reference only UNIQUE (including PRIMARY) and NOT NULL keys.
Foreign keys should reference only a column (or columns) that have been declared either PRIMARY KEY or NOT NULL UNIQUE. If you think you need a foreign key reference to a nonunique column or columns, that's a big red flag, waving in your face, held by a guy yelling, "You have a design problem!"
ALTER TABLE `groups` ADD FOREIGN KEY ( `company_id` ) REFERENCES `summaries`.`companies` (
`id`
) ON DELETE CASCADE ;
MySQL said:
#1452 - Cannot add or update a child row: a foreign key constraint fails (`summaries/#sql-164a_33c`, CONSTRAINT `#sql-164a_33c_ibfk_1` FOREIGN KEY (`company_id`) REFERENCES `companies` (`id`) ON DELETE CASCADE)
companies.id is primary auto increment int(11)
company_id is index int(11)
I don't understand the error message. Can anyone shed some light on this?
That means you have at least one row in the child table that references a non-existent row in the parent table.
If you are absolutely sure that you are okay with having a data integrity issue like that, you can add the foreign key by disabling foreign key checks before you run the ALTER TABLE command:
SET FOREIGN_KEY_CHECKS = 0;
I just had this problem, although in a somewhat more specific scenario.
In my case, I had added, to an existing table, a column that I needed to be both nullable and act as a foreign key (i.e., for non-null entries to be bound by a foreign key constraint).
The referenced column looked like this:
+-------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+----------------+
| id | int(10) | NO | PRI | NULL | auto_increment |
+-------------+-------------+------+-----+---------+----------------+
and the referencing one like this:
+-------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+----------------+
| bed_id | int(10) | YES | | NULL | |
+-------------+-------------+------+-----+---------+----------------+
Turned out that I had forgotten to specify DEFAULT NULL when adding the referencing column to the existing table and so it was automatically filled with zeros, which failed the foreign key constraint.
I changed them to NULL:
update devices set bed_id = NULL where bed_id = 0;
and then successfully added the foreign key constraint. Hope this helps someone
It looks like it tried to copy the groups table to a temp table in the summaries database.
MySQL tried to put the requested constraints on the temp table first. There may possibly be one or more rows in the groups table (hence the temp table also) whose company_id is no longer present in the summaries.companies table.
To verfiy : Try running a LEFT JOIN between groups and summaries.companies WHERE companies.id is NULL. If you get back any rows from that LEFT JOIN, you found the bad rows in the groups table.
Give it a Try !!!
Checks that "companies" table is not empty,if is empty and you have no data at moment.
set SET FOREIGN_KEY_CHECKS = 0;
like Ike said you before.