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create table connections(user_id int, sender_id int, receiver_id int, status int);
create table users(id int);
SELECT users.*, (SELECT sender_id FROM connections WHERE receiver_id = users.id ) 1_level FROM users;
SELECT users.*, (SELECT sender_id FROM (SELECT receiver_id as sender_id FROM connections WHERE sender_id = users.id ) 2_level ) 1_level FROM users;
Unknown column 'users.id' in 'where clause'
SELECT users.*, (SELECT COUNT(user_id) AS mutual_connection FROM (SELECT user_id FROM (SELECT sender_id AS user_id FROM connections WHERE status = '2' UNION SELECT receiver_id AS user_id FROM connections WHERE status = '2' ) tempUser WHERE user_id IN (SELECT sender_id AS user_id FROM connections WHERE receiver_id = '4' AND status = '2' UNION SELECT receiver_id AS user_id FROM connections WHERE sender_id = '4' AND status= '2') GROUP BY user_id) as mutualConnection WHERE user_id = users.id #<<<-------------- Here ) uid FROM users