mysql: two tables but three select criteria
SELECT id FROM users, user_depts
WHERE users.id = user_depts.uid
AND user_depts.uid = 7

SELECT uid FROM user_depts
WHERE EXISTS (
  SELECT * FROM user_depts a
  WHERE uid = 7 AND a.did = user_depts.did
)

select users.id from users
inner join user_depts.uid = users.id
where users.id = 7