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
There's no need to group in that subquery... a where clause would suffice:
SELECT * FROM tblpm n
WHERE date_updated=(SELECT MAX(date_updated)
FROM tblpm WHERE control_number=n.control_number)
Also, do you have an index on the 'date_updated' column? That would certainly help.
Fast and easy with HAVING:
SELECT * FROM tblpm n
FROM tblpm GROUP BY control_number
HAVING date_updated=MAX(date_updated);
In the context of HAVING
, MAX
finds the max of each group. Only the latest entry in each group will satisfy date_updated=max(date_updated)
. If there's a tie for latest within a group, both will pass the HAVING
filter, but GROUP BY
means that only one will appear in the returned table.
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);
Another way that doesn't use group by:
SELECT * FROM tblpm n
WHERE date_updated=(SELECT date_updated FROM tblpm n
ORDER BY date_updated desc LIMIT 1)