Question

I have a table like this:

user_id | wants_user_id | state

And I want to select rows which are mutual and have a state column with value “yes” or “maybe”.

Example data:

Row 1: 1 | 2 | "yes"
Row 2: 2 | 1 | "maybe"
Row 3: 3 | 2 | "yes"

In this case I would like to select row 1 and 2 and on wants_user_id (second column) I would like to join user data from table users, but I can’t figure out the sql command.

Thank you for your help.

Answer 1

Use a self-join to get matching rows (I’m assuming “mutual rows” here means users that want each other):

SELECT *
FROM myTable a
INNER JOIN myTable b ON a.user_id = b.wants_user_id
    AND b.user_id = a.wants_user_id
INNER JOIN users ua ON a.user_id = ua.id
INNER JOIN users ub ON b.user_id = ub.id
WHERE a.state IN ('yes', 'maybe')
AND b.state IN ('yes', 'maybe')

In this case, you will get two “duplicate” rows, one for 1<->2 and one for 2<->1. To eliminate this, you can add a AND b.user_id > a.user_id to the join condition – this will select only one row of each pair.