Is this a valid MySQL query?

Question

I've got 4 tables: users, messages, files, notes. I want to make a query which will delete all of a users' stuff if he's deleted. Will the following work:

DELETE FROM users, messages, files, notes WHERE userId = '$userId'

All 4 tables have got the column userId

Answer 1

You can split the statements in to four separate statements, in order to delete the rows associated with the userid.

DELETE FROM users WHERE userId = '$userId'
DELETE FROM messages WHERE userId = '$userId'
DELETE FROM files WHERE userId = '$userId'
DELETE FROM notes WHERE userId = '$userId'

The other alternative would be to cascade the deletes across the foreign keys.

When you create a foreign key you can choose what you want to do to the child records on the deletion of a parent record. This is denoted below by specifying the ON Delete in the creation of the foreign key. Here is some reference http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html

[CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

Answer 2

DELETE  u, m, f, p
FROM    users u
LEFT JOIN
        messages m
ON      m.user_id = u.id
LEFT JOIN
        files f
ON      f.user_id = u.id
LEFT JOIN
        posts p
ON      p.user_id = u.id
WHERE   u.id = 1

This assumes that the record with id = 1 exists in users, otherwise nothing will be deleted.

Note, however, that this is not very efficient and four separate deletes will work better.