MySQL Error Code 1215: “Cannot add foreign key constraint”

MySQL Error Code 1215

Updated 7-05-2019

In this blog, we’ll look at how to resolve MySQL error code 1215: “Cannot add foreign key constraint”.

Our Support customers often come to us with things like “My database deployment fails with error 1215”, “Am trying to create a foreign key and can’t get it working” or “Why am I unable to create a constraint?” To be honest, the error message doesn’t help much. You just get the following line:

ERROR 1215 (HY000): Cannot add foreign key constraint

There’s actually a multitude of reasons this can happen, and in this blog post is a compendium of the most common reasons why you can get MySQL Error Code 1215, how to diagnose your case to find which one is affecting you, and potential solutions for adding the foreign key.

(Note: be careful when applying the proposed solutions, as many involve ALTERing the parent table and that can take a long time blocking the table, depending on your table size, MySQL version and the specific ALTER operation being applied; In many cases using pt-online-schema-change will be likely a good idea).

So, onto the solutions:

The best way to start investigating this error is by getting more information about it from LATEST FOREIGN KEY ERROR section of SHOW ENGINE INNODB STATUS. This will give you a hint regarding the problem, which should help you identify your case in the list below.

1) The table or index the constraint refers to does not exist yet (usual when loading dumps).

How to diagnose: Run SHOW TABLES or SHOW CREATE TABLE for each of the parent tables. If you get error 1146 for any of them, it means tables are being created in the wrong order.
How to fix: Run the missing CREATE TABLE and try again, or temporarily disable foreign-key-checks. This is especially needed during backup restores where circular references might exist. Simply run:

Example:

2) The table or index in the constraint references misuses quotes.

How to diagnose: Inspect each FOREIGN KEY declaration and make sure you either have no quotes around object qualifiers or that you have quotes around the table and a SEPARATE pair of quotes around the column name.
How to fix: Either don’t quote anything or quote the table and the column separately.
Example:

3) The local key, foreign table or column in the constraint references have a typo:

How to diagnose: Run SHOW TABLES and SHOW COLUMNS and compare strings with those in your REFERENCES declaration.
How to fix: Fix the typo once you find it.
Example:

4) The column the constraint refers to is not of the same type or width as the foreign column:

How to diagnose: Use SHOW CREATE TABLE parent to check that the local column and the referenced column both have the same data type and width.
How to fix: Edit your DDL statement such that the column definition in the child table matches that of the parent table.
Example:

5) The foreign object is not a KEY of any kind

How to diagnose: Use SHOW CREATE TABLE parent to check that if the REFERENCES part points to a column, it is not indexed in any way.
How to fix: Make the column a KEY, UNIQUE KEY or PRIMARY KEY on the parent.
Example:

6) The foreign key is a multi-column PK or UK, where the referenced column is not the leftmost one

How to diagnose: Do a SHOW CREATE TABLE parent to check if the REFERENCES part points to a column that is present in some multi-column index(es) but is not the leftmost one in its definition.
How to fix: Add an index on the parent table where the referenced column is the leftmost (or only) column.
Example:

7) Different charsets/collations among the two table/columns

How to diagnose: Run SHOW CREATE TABLE parent and compare that the child column (and table) CHARACTER SET and COLLATE parts match those of the parent table.
How to fix: Modify the child table DDL so that it matches the character set and collation of the parent table/column (or ALTER the parent table to match the child’s wanted definition.
Example:

8) The parent table is not using InnoDB

How to diagnose: Run SHOW CREATE TABLE parent and verify if ENGINE=INNODB or not.
How to fix: ALTER the parent table to change the engine to InnoDB.
Example: