Using Case On Query

Question

I want to get value from table chat using case, like this :

SELECT user_one, user_two
FROM chatting, users
WHERE
CASE 
WHEN user_one = 36 
THEN user_two = 55
WHEN user_two = 55 
THEN user_one = 36 
END

but i get this error after "THEN user_two = 55" The error is "Incorrect syntax near '='."

Answer 1

It is not possible to make a logical expression the predicate of a CASE expression. That is, what follows THEN and ELSE has to be literal values. But, we can rewrite your logic as:

SELECT
    user_one,
    user_two
FROM chatting c
INNER JOIN users u
    -- ON <some condition>
WHERE
    (user_one = 36 AND user_two = 55) OR (user_one = 55 AND user_two = 36);

Depending on your version of SQL, we might be able to write the WHERE clause using tuples, as:

WHERE
    (user_one, user_two) IN ((36, 55), (55, 36));