Is this a valid MySQL query?

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开发者_如何学运维, notes WHERE userId = '$userId'

All 4 tables have got the column userId

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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]

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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.

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in the where statement, if you have more than one table reference, you should refer to TABLE.COLUMN, eg : WHERE users.userid = '$userId'. I recommend to do 1 sentence per table

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In your WHERE, you have userId = '$userId'. Other people can correct me if I'm wrong, but I'm pretty sure that the userId would be ambiguous and this statement wouldn't run. @John Hartsocks answer should work though if that is indeed the case.

That is besides the point though. If you're worried about making sure a user is completely deleted, then you should be using foreign key constraints. You said when you delete a user, you want to make sure that all information pertaining to that user is deleted. If you add the FK constraint to every table that has a user id (besides the users table, in which case it would have the primary key constraint), it should not be possible for you to delete a user, without first making sure all other data referencing that user is deleted.

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Yes.

http://dev.mysql.com/doc/refman/5.0/en/delete.html