Question

I have two tables

requests - request_id, request_message, user_id
responses - response_id, request_id, response_message, user_id, status

I am running this query to get the count of unread responses, which belongs to a particular user.

SELECT COUNT(*) 
FROM RESPONSES 
WHERE status = 'U' 
and request_id IN ( SELECT request_id FROM requests WHERE user_id  = '$User_id')

Is there any way to optimize it using join??

Answer 1

I’d suggest using an explicit join instead of a subquery:

SELECT 
 count(*) 
FROM
  responses res
INNER JOIN
  requests req
ON
  req.request_id=res.request_id
WHERE 
  req.user_id='$User_id'
  and res.status='U';

are the request_id fields indexed? I’m guessing it’s the PKEY in requests. What about the user_id field in requests?

Also, while I copied your syntax for it, I hope $User_id is not a variable being filled by user generated input. You should be using bind parameters (the method will depend on what language you are using.)