Question

I want to use sql JOIN with COUNT in a way that using the COUNT as the condition for the JOIN.

e.g.

SELECT * FROM tbl1
  INNER JOIN (SELECT * FROM tbl2) t2
  ON (SELECT COUNT(*) FROM tbl1) > 0

Is this possible or can someone please tell me another way?

PS- The real question I’m having is that I have two tables A and B and I need to select id from A where the date of creation is greater than some value. At the same time I want to make sure that table B does not have that id in table A. This works fine but when I have no data in table B, the query gives no result.

SELECT a.user_id from (SELECT * FROM A WHERE DATEDIFF(event_date,'certain_value') >=another_value) a INNER JOIN B b ON a.user_id != b.user_id

I tried to use count like this but failed

SELECT a.user_id from (SELECT * FROM A WHERE DATEDIFF(event_date,'certain_value') >=another_value) a INNER JOIN B b ON count(B.user_id) = 0 OR a.user_id != b.user_id

Answer 1

The way your query is written, condition in ON is doing nothing and equivalent to ON (1=1) (if there is no records in tbl1, select * from tbl1 inner join tbl2 on [any condition] will always return empty resultset).

From your updated question I think you need:

SELECT
    a.user_id
FROM
    (SELECT *
     FROM   A
     WHERE  DATEDIFF(A.event_date, 'certain_value') >= another_value) AS a
WHERE
    NOT EXISTS (SELECT 1
                FROM   b
                WHERE  b.user_id = a.user_id)