MySQL添加外键

Question

I want to update a column which is currently a plain INT(16) so that it references a FK on another table. I've tried the following, but with errors:

ALTER TABLE ts_keys ADD CONSTRAINT FK_account_id FOREIGN KEY (account_id) REFERENCES accounts(id) ON UPDATE CASCADE ON DELETE CASCADE

EDIT: Sorry, forgot to add the error:

Can't create table (errno: 150)

Both tables are Innodb.

EDIT 2: I also tried re-creating the table but same error:

    CREATE TABLE ts_keys (
  id int PRIMARY KEY AUTO_INCREMENT,
  account_id int,
  FOREIGN KEY fk_account_id1(account_id) REFERENCES accounts(id)
) ENGINE=InnoDB;

Answer 1

The datatype of the foreign key column must match EXACTLY the datatype of the referenced column.

Do a SHOW CREATE TABLE accounts and look at the definition of the id column.

Whatever that column is defined as INT UNSIGNED , BIGINT , VARCHAR(16) , whatever,

the column you want to define as a foreign key (the account_id column in ts_keys table) must match that datatype EXACTLY . (It's just the datatype that has to match. The column comment doesn't have to match, the DEFAULT value doesn't have to match, the NULL/NOT NULL doesn't have to match. But it's required that the datatypes match.

---

Your syntax for adding the constraint looks correct:

ALTER TABLE ts_keys
  ADD CONSTRAINT FK_account_id 
  FOREIGN KEY (account_id) 
  REFERENCES accounts(id) 
  ON UPDATE CASCADE ON DELETE CASCADE

Admittedly, the "Can't create table (errno: 150)" has to be the least helpful message regarding what's actually causing the problem. (At least the error isn't the "check the manual" syntax error.