Question

I wrote this query :

SELECT * 
FROM etape_prospection INNER JOIN type_prospection 
ON etape_prospection.type_prosp_id = type_prospection.type_prosp_id 
WHERE etape_prospection.prosp_id IN (select transfert.prosp_id 
                                     from transfert 
                                     where transfert.user_code ='3' 
                                     AND transfert.date_transfert='2012-01-20');

My boss doesn’t like this query because of the IN keyword which makes a full table scan , so how to rewrite it without the IN keyword ?

Answer 1

You can use EXISTS instead of IN this is usually cheaper than IN and sometimes also cheaper than the INNER JOIN suggested in other answers:

SELECT * 
FROM etape_prospection INNER JOIN type_prospection 
ON etape_prospection.type_prosp_id = type_prospection.type_prosp_id 
WHERE EXISTS (select 1
               FROM transfert 
               WHERE transfert.prosp_id = etape_prospection.prosp_id
               AND transfert.user_code ='3' 
               AND transfert.date_transfert='2012-01-20');

Also (reminded by the comment by danihp on one of the other answers): using INNER JOIN can have the unwanted/unintended side effect of duplicating rows in the resultset if there are multiple rows in transfert that satisfy these conditions.