WHERE is used to filter data before aggregation HAVING is used to filter data after aggregation (after GROUP BY clause) As far as I remembered, you can actually use both WHERE and HAVING at the same time. Just note that WHERE will filter the data before the aggregation. SELECT staffNo, COUNT(salary) FROM Staff; This is wrong because you have regular column (staffNo) and then COUNT(salary) which is an aggregation function and the query have no GROUP BY clause for the regular column (staffNo) SELECT COUNT(*) AS myCount FROM PropertyForRent WHERE rent > 350; This is correct, you are doing WHERE to filter the data before aggregating it. And then you only select COUNT(*) which is an aggregation function and no regular column. SELECT COUNT(staffNo) AS myCount, SUM(salary) AS mySum FROM Staff WHERE position = ‘Manager’; This is also correct, because you are only selecting COUNT(staffNo) and SUM(salary) which are both aggregation function. See there are no regular column involved. SELECT staffName, COUNT(staffNo) AS myCount, SUM(salary) AS mySum FROM Staff WHERE position = 'Manager'; This one however is incorrect because you have staffName column (regular column) selected and 2 aggregation. To correct it simply do GROUP BY clause to staffName SELECT staffName, COUNT(staffNo) AS myCount, SUM(salary) AS mySum FROM Staff WHERE position = 'Manager' GROUP BY staffName; But if I am not mistaken you are allowed to do this SELECT position, COUNT(staffNo) AS myCount, SUM(salary) AS mySum FROM Staff WHERE position = 'Manager'; Because you are doing filtering to the position column, then all the rows that remain after the WHERE clause filtering have the same position, so there won't be any need for a GROUP BY clause. But I'm not really sure about this so it's better if you give it a try and maybe learn something new!