Question

I have two tables T1L and T2U. For records matching the left join, if Deleted!=0 I don’t want those records from either tables.

SELECT
    IF(U.Column1 IS NULL, L.id, -U.id) AS id,
    IF(U.Column1 IS NULL, L.Column1, U.Column1) AS Column1,
    IF(U.Column1 IS NULL, L.Column2, U.Column2) AS Column2,
    IF(U.Column1 IS NULL, L.Deleted, U.Deleted) AS Deleted
FROM
    T1 L
LEFT JOIN T2 U
    ON L.Column1 = U.Column1 AND L.Column2=U.Column2 AND U.Deleted=0

If Deleted!=0, currently it will give me the record from TI. I don’t want that record at all from either TI or T2.

How can I modify the query to achieve that?

Answer 1

Move it to the WHERE clause and add an additional check for NULL.

SELECT
    IF(U.Column1 IS NULL, L.id, -U.id) AS id,
    IF(U.Column1 IS NULL, L.Column1, U.Column1) AS Column1,
    IF(U.Column1 IS NULL, L.Column2, U.Column2) AS Column2,
    IF(U.Column1 IS NULL, L.Deleted, U.Deleted) AS Deleted
FROM
    T1 L
LEFT JOIN T2 U
    ON L.Column1 = U.Column1 AND L.Column2=U.Column2 
WHERE
    U.Deleted=0
    OR U.Deleted IS NULL