MySQL: Use CASE/ELSE value as join parameter

Question

I'm trying to join the NAME and PHOTO from USERS table to the TRANSACTIONS table based on who is the payer or payee. It keeps telling me can't find the table this -- What am I doing wrong?

SELECT `name`,`photo`,`amount`,`comment`,
(
    CASE `payer_id`
    WHEN 72823 THEN `payee_id`
    ELSE `payer_id`
    END
) AS `this`
FROM `transactions`
RIGHT JOIN `users` ON (`users`.`id`=`this`)
WHERE `payee_id`=72823 OR `payer_id`=72823

Answer 1

From the documentation about aliases:

The alias is used as the expression's column name and can be used in GROUP BY, ORDER BY, or HAVING clauses.

You can't use an alias in a join. You can use it only in the places listed above. The reason is that the alias is on a field in the result of the join. If the join were allowed to these aliases in its definition it would (or could) result in recursive definitions.

To solve your problem you could repeat the CASE clause in both places:

SELECT `name`,`photo`,`amount`,`comment`,
(
    CASE `payer_id`
    WHEN 72823 THEN `payee_id`
    ELSE `payer_id`
    END
) AS `this`
FROM `transactions`
RIGHT JOIN `users` ON `users`.`id`= (
    CASE `payer_id`
    WHEN 72823 THEN `payee_id`
    ELSE `payer_id`
    END
)
WHERE `payee_id`=72823 OR `payer_id`=72823

However I would probably rewrite this query as two selects and UNION them:

SELECT name, photo, amount, comment, payer_id AS this
FROM transactions
JOIN users ON users.id = payer_id
WHERE payee_id = 72823
UNION ALL
SELECT name, photo, amount, comment, payee_id AS this
FROM transactions
JOIN users ON users.id = payee_id
WHERE payer_id = 72823

Result:

'name3', 'photo3', 30, 'comment3', 3
'name1', 'photo1', 10, 'comment1', 1
'name2', 'photo2', 20, 'comment2', 2

Test data:

CREATE TABLE users (id INT NOT NULL, name NVARCHAR(100) NOT NULL, photo NVARCHAR(100) NOT NULL);
INSERT INTO users (id, name, photo) VALUES
(1, 'name1', 'photo1'),
(2, 'name2', 'photo2'),
(3, 'name3', 'photo3'),
(4, 'name4', 'photo4');

CREATE TABLE transactions (amount INT NOT NULL, comment NVARCHAR(100) NOT NULL, payer_id INT NOT NULL, payee_id INT NOT NULL);
INSERT INTO transactions (amount, comment, payer_id, payee_id) VALUES
(10, 'comment1', 72823, 1),
(20, 'comment2', 72823, 2),
(30, 'comment3', 3, 72823),
(40, 'comment4', 4, 5);