MySQL cannot create foreign key constraint

Question

I\'m having some problems creating a foreign key to an existing table in a mysql database.

I have the table exp:

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Answer 1

The exact order of the primary key also needs to match with no extra columns in between.

I had a primary key setup where the column order actually matches, but the problem was the primary key had an extra column in it that is not part of the foreign key of the referencing table

e.g.) table 2, column (a, b, c) -> table 1, column (a, b, d, c) -- THIS FAILS

I had to reorder the primary key columns so that not only they're ordered the same way, but have no extra columns in the middle:

e.g.) table 2, column (a, b, c) -> table 1, column (a, b, c, d) -- THIS SUCCEEDS

Answer 2

In my case, it turned out the referenced column wasn't declared primary or unique.

https://stackoverflow.com/a/18435114/1763217

Answer 3

This error can also occur, if the references table and the current table don't have the same character set.

Answer 4

Just throwing this into the mix of possible causes, I ran into this when the referencing table column had the same "type" but did not have the same signing.

In my case, the referenced table colum was TINYINT UNSIGNED and my referencing table column was TINYINT SIGNED. Aligning both columns solved the issue.

Answer 5

According to http://dev.mysql.com/doc/refman/5.5/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. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order.

InnoDB permits a foreign key to reference any index column or group of columns. However, in the referenced table, there must be an index where the referenced columns are listed as the first columns in the same order.

So if the index in referenced table is exist and it is consists from several columns, and desired column is not first, the error shall be occurred.

The cause of our error was due to violation of following rule:

Corresponding columns in the foreign key and the referenced key must have similar data types. The size and sign of integer types must be the same. The length of string types need not be the same. For nonbinary (character) string columns, the character set and collation must be the same.

Answer 6

As mentioned @Anton, this could be because of the different data type. In my case I had primary key BIGINT(20) and tried to set foreight key with INT(10)