SELECT users.user_id, admin.permission
FROM users
INNER JOIN admin
ON users.user_id = admin.user_id
WHERE users.user_id = 2;

SELECT users.user_id, admin.permission, users.name, users.birth_date
FROM users
LEFT JOIN admin
ON users.user_id = admin.user_id;


SELECT users.user_id, admin.permission, users.name, users.birth_date
FROM users
RIGHT JOIN admin
ON users.user_id = admin.user_id;

SELECT name FROM users UNION ALL SELECT permission FROM admin;