Question

Thanks for the great answers!

For More Information

• More info on MySQL IN comparison operator
• Joining Tables
• Subqueries with ANY, IN, and SOME

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This is hard to explain, so lets set the stage…

userActions         userGroupMap +------+--------+   +------+-------+ | user | action |   | user | group | +------+--------+   +------+-------+ | x    | acted! |   | x    | a     | | y    | acted! |   | y    | a     | | y    | acted! |   | z    | b     | | z    | acted! |   +------+-------+ | y    | acted! | | z    | acted! | | x    | acted! | | z    | acted! | +------+--------+ 

I want to select group a’s actions. My idea was to

SELECT actions, user FROM userActions     WHERE user = (SELECT user, group FROM userGroupMap WHERE group = a) 

But obviously this subquery returns more than one row. Should I use a JOIN?

Subquery returns more than 1 row 

Answer 1

One approach is this:

SELECT actions,        user FROM   userActions WHERE  user IN                (SELECT user                FROM    userGroupMap                WHERE   [group] = 'a'                ); 

However, with large tables, this query tends to be inefficient and doing a join is better:

SELECT actions,        userActions.user FROM   userActions        INNER JOIN               (SELECT user               FROM    userGroupMap               WHERE   [group] = 'a'               ) AS tmp        ON     userActions.user = tmp.user; 

Alternatively, as Jonathon mentioned, you could have done this and its pretty much as efficient, if not more:

SELECT actions,        userActions.user FROM   userActions        INNER JOIN userGroupMap        ON     userActions.user = userGroupMap.user WHERE  [group] = 'a';