Reputation: 2664

MySQL : ERROR 1215 (HY000): Cannot add foreign key constraint

I have read Database system concepts, 6th edition, Silberschatz. I'm going to implement the university database system shown in chapter 2 on OS X on MySQL. But I have a trouble with creating the table course. the table department looks like

mysql> select * from department
    -> ;
+------------+----------+-----------+
| dept_name  | building | budget    |
+------------+----------+-----------+
| Biology    | Watson   |  90000.00 |
| Comp. Sci. | Taylor   | 100000.00 |
| Elec. Eng. | Taylor   |  85000.00 |
| Finance    | Painter  | 120000.00 |
| History    | Painter  |  50000.00 |
| Music      | Packard  |  80000.00 |
| Physics    | Watson   |  70000.00 |
+------------+----------+-----------+

mysql> show columns from department
    -> ;
+-----------+---------------+------+-----+---------+-------+
| Field     | Type          | Null | Key | Default | Extra |
+-----------+---------------+------+-----+---------+-------+
| dept_name | varchar(20)   | NO   | PRI |         |       |
| building  | varchar(15)   | YES  |     | NULL    |       |
| budget    | decimal(12,2) | YES  |     | NULL    |       |
+-----------+---------------+------+-----+---------+-------+

Creating the table course causes the following error.

mysql> create table course
    -> (course_id varchar(7),
    -> title varchar (50),
    -> dept_name varchar(20),
    -> credits numeric(2,0),
    -> primary key(course_id),
    -> foreign key (dept_name) references department);
ERROR 1215 (HY000): Cannot add foreign key constraint

after searching google for foreign key constraint, I have just learned that the word 'foreign key constraint' indicates that data from foreign key column in the table course must exist in primary key column in the table department. But I should have met this error when inserting data.

If not, why does author make me execute that SQL statement?

If I really execute erroneous SQL statement, Does I have to designate dept_name in course table as foreign key after inserting some data?

EDIT : typing set foreign_key_checks=0 into mysql> does not fix the error.

------------------------
LATEST FOREIGN KEY ERROR
------------------------
2013-09-21 16:02:20 132cbe000 Error in foreign key constraint of table university/course:
foreign key (dept_name) references department):
Syntax error close to:
)
mysql> set foreign_key_checks=0
    -> ;
Query OK, 0 rows affected (0.00 sec)
mysql> create table course
    -> (course_id varchar(7),
    -> title varchar(50),
    -> dept_name varchar(20),
    -> credits numeric(2,0),
    -> primary key(course_id),
    -> foreign key (dept_name) references department);
ERROR 1215 (HY000): Cannot add foreign key constraint

Upvotes: 73

Answers (19)

Roberto Góes

Reputation: 828

Because related tables should have the same config about CHARSET and ENGINE. Execute:

show table status from <DATABASE_NAME>;

See Engine and Collation about your table. When you create put after ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Upvotes: 0

Fabrício José Souza

Reputation: 26

Be aware it can be several different things.

In my case it was the table/field collation:

FK target: CHARSET=utf8 COLLATE=utf8_unicode_ci
FK source field: CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

Upvotes: 0

Anderson Barros

Reputation: 11

In my case the engine is different between the parent table and the child table. Making the engines equal is working

The problem: parent table 'engine' => 'MyISAM', child table 'engine' => 'innoDB',

Correção: parent table 'engine' => 'MyISAM', child table 'engine' => 'MyISAM',

Upvotes: 1

Harm

Reputation: 777

I had this error when I tried to import (in MysqlWorkbench) from a PhpAdminMySQL export. After verifying I had disabled the unique keys and foreign keys with:

SET unique_checks=0;
SET foreign_key_checks = 0;

I still get the same error (MySQL : ERROR 1215 (HY000): Cannot add foreign key constraint). The error occurred on this create statement.

DROP TABLE IF EXISTS `f1_pool`;
CREATE TABLE `f1_pool` (
 `id` int(11) NOT NULL,
 `name` varchar(45) NOT NULL,
 `description` varchar(45) DEFAULT NULL COMMENT 'Optional',
 `ownerId` int(11) NOT NULL,
 `lastmodified` timestamp NOT NULL DEFAULT current_timestamp() ON UPDATE current_timestamp()
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

No foreign key or unique index, so what is wrong here? Finally (after 90 minutes puzzling) I decided to restart MySQL and do the import again with one modification: I dropped all tables before doing the import. And there was no error, all functioned, tables and views restored. So my advice, if all looks ok, first try to restart MySQL!

Upvotes: 1

DEV

Reputation: 2176

In my case charset, datatype every thing was correct. After investigation I found that in parent table there was no index on foreign key column. Once added problem got solved.

Upvotes: 0

Andath

Reputation: 22724

Even if this is not directly linked precisely to your situation, it may help further readers to note that you can get exactly the same error output when you type show engine innodb mstatus if you do not respect the order of creating the database tables; meaning you must not add a foreign constraint referencing a table that does not exist yet. The reference table must exist prior to the table which points to it.

This is also true when the table creation order is respected but not the columns involved in the foreign key constraint.

Upvotes: 0

Shiyao

Reputation: 36

I came across the same issue as well. Not sure why this is working but it indeed works: Try add ENGINE INNODB after your create query.

mysql> create table course
-> (course_id varchar(7),
-> title varchar (50),
-> dept_name varchar(20),
-> credits numeric(2,0),
-> primary key(course_id),
-> foreign key (dept_name) references department) ENGINE INNODB;

Upvotes: 0

EdgeCaseBerg

Reputation: 2841

I don't see anyone stating this explicitly and I had this same error message and my problem was that I was trying to add a foreign key to a TEMPORARY table. Which is disallowed as noted in the manual

Foreign key relationships involve a parent table that holds the central data values, and a child table with identical values pointing back to its parent. The FOREIGN KEY clause is specified in the child table. The parent and child tables must use the same storage engine. They must not be TEMPORARY tables.

(emphasis mine)

Upvotes: 0

g10guang

Reputation: 5275

I don't meet the problem as you. But I get the same ERROR Message. So I mark it down here for others' convience.

Check the charset of two table if the column type is char or varchar. I use a charset=gbk, but I create a new table whose default charset=utf8. So the charset is not the same.

ERROR 1215 (HY000): Cannot add foreign key constraint

To solve it is to use the same charset. For example utf8.

Upvotes: 3

anjaneyulubatta505

Reputation: 11665

Below code worked for me

set @@foreign_key_checks=0;
ALTER TABLE  `table1` ADD CONSTRAINT `table1_fk1` FOREIGN KEY (`coloumn`) REFERENCES `table2` (`id`) ON DELETE CASCADE;

Upvotes: 1

Gray

Reputation: 116878

ERROR 1215 (HY000): Cannot add foreign key constraint

It is also worth noting that you get this error when the type of the column that is a foreign key in another able doesn't explicitly match the column in the correct table.

For example:

alter table schoolPersons
         add index FKEF5AB5E532C8FBFA (student_id),
         add constraint FKEF5AB5E532C8FBFA
         foreign key (student_id)
         references student (id);
ERROR 1215 (HY000): Cannot add foreign key constraint

This was because the student_id field was defined as:

mysql> desc schoolPersons;
+--------------------+------------+------+-----+---------+----------------+
| Field              | Type       | Null | Key | Default | Extra          |
+--------------------+------------+------+-----+---------+----------------+
| student_id         | bigint(20) | YES  |     | NULL    |                |

while the id field in the student table was defined as:

mysql> desc persons;
+--------------+----------------------+------+-----+-------------------+-----------------+
| Field        | Type                 | Null | Key | Default           | Extra           |
+--------------+----------------------+------+-----+-------------------+-----------------+
| id           | int(10) unsigned     | NO   | PRI | NULL              | auto_increment  |

The bigint(20) (generated from Java long by hibernate) is not compatible with int(10) unsigned (Java int).

Upvotes: 5

vedavyasa k

Reputation: 1661

Just add 'unsigned' for the FOREIGN constraint

`FK` int(11) unsigned DEFAULT NULL,

Upvotes: 2

Jack Davidson

Reputation: 4933

It is also possible to get this error if the foreign key is not a primary key within its own table.

I did an ALTER TABLE and accidentally removed the primary key status of a column, and got this error.

Upvotes: 5

naube

Reputation: 1

CONSTRAINT vendor_tbfk_1 FOREIGN KEY (V_CODE) REFERENCES vendor (V_CODE) ON UPDATE CASCADE

this is how it could be... look at the referencing column part. (V_code)

Upvotes: -1

am0wa

Reputation: 8377

When you get this vague error message, you can find out the more specific error by running

SHOW ENGINE INNODB STATUS;

The most common reasons are that when creating a foreign key, both the referenced field and the foreign key field need to match:

Engine should be the same e.g. InnoDB
Datatype should be the same, and with same length.
e.g. VARCHAR(20) or INT(10) UNSIGNED
Collation should be the same. e.g. utf8
Unique - Foreign key should refer to field that is unique (usually private) in the reference table.

Another cause of this error is:
You have defined a SET NULL condition though some of the columns are defined as NOT NULL.

Upvotes: 102

Alkanshel

Reputation: 4429

It's worth noting that this error can also happen if the target table or column you're using in the REFERENCES portion simply doesn't exist.

Upvotes: 1

workflo

Reputation: 351

Maybe your dept_name columns have different charsets.

You could try to alter one or both of them:

ALTER TABLE department MODIFY dept_name VARCHAR(20) CHARACTER SET utf8;
ALTER TABLE course MODIFY dept_name VARCHAR(20) CHARACTER SET utf8;

Upvotes: 24

Faishal

Reputation: 1493

The syntax of FOREIGN KEY for CREATE TABLE is structured as follows:

FOREIGN KEY (index_col_name)
        REFERENCES table_name (index_col_name,...)

So your MySQL DDL should be:

 create table course (
        course_id varchar(7),
        title varchar(50),
        dept_name varchar(20),
        credits numeric(2 , 0 ),
        primary key (course_id),
        FOREIGN KEY (dept_name)
            REFERENCES department (dept_name)
    );

Also, in the department table dept_name should be VARCHAR(20)

More information can be found in the MySQL documentation

Upvotes: 57

ta.speot.is

Reputation: 27214

foreign key (dept_name) references department

This syntax is not valid for MySQL. It should instead be:

foreign key (dept_name) references department(dept_name)

MySQL requires dept_name to be used twice. Once to define the foreign column, and once to define the primary column.

13.1.17.2. Using FOREIGN KEY Constraints

... [the] essential syntax for a foreign key constraint definition in a CREATE TABLE or ALTER TABLE statement looks like this:
[CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name, ...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION

Upvotes: 6

MySQL : ERROR 1215 (HY000): Cannot add foreign key constraint

Answers (19)

13.1.17.2. Using FOREIGN KEY Constraints

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