GROUP BY having MAX date

Question

I have problem when executing this code:

SELECT * FROM tblpm n 
WHERE date_updated=(SELECT MAX(date_updated) 
FROM tblpm GROUP BY control_number 
HAVING cont         


        

Answer 1

Putting the subquery in the WHERE clause and restricting it to n.control_number means it runs the subquery many times. This is called a correlated subquery, and it's often a performance killer.

It's better to run the subquery once, in the FROM clause, to get the max date per control number.

SELECT n.* 
FROM tblpm n 
INNER JOIN (
  SELECT control_number, MAX(date_updated) AS date_updated
  FROM tblpm GROUP BY control_number
) AS max USING (control_number, date_updated);