Question

Three tables, columns as follows:

A:   A_id, B_id, C_id, flag, ...
B:   B_id, date, ...
C:   C_id, date

If A.flag is NULL, then I need all rows from A joined with B on B_id that have B.date in the past.

If A.flag is not NULL, then I need all rows from A joined with B on B_id that have C.date in the past, C being joined on C_id.

An attempt:

SELECT *
FROM A, B, C
WHERE A.A_id = B.B_id
AND ((A.flag IS NULL AND (NOW() > B.date) OR
     (A.flag IS NOT NULL AND (NOW() > C.date) AND C.C_id = A.C_id))

But I need some condition in the A.flag is NULLline to stop it joining with each row from C. That is what I can’t work out.

Or is there an easier way to do this?

Answer 1

You could try

SELECT a.*, b.* 
FROM a INNER JOIN b ON a.B_id = b.B_id
WHERE a.flag IS NULL AND b.date < NOW()
UNION
SELECT a.*, b.* 
FROM a INNER JOIN b ON a.B_id = b.B_id
INNER JOIN c ON a.C_id = c.C_id
WHERE a.flag IS NOT NULL AND c.date < NOW()