How do add together a COUNT() using SUM()? - mysql

Question

Basically, I have this -
SELECT COUNT(student_dormpm.DormCode) * dorm_datapm.DormCharge
FROM student_dormpm
JOIN dorm_datapm
USING (DormCode)
GROUP BY DormCode;
with a result of
+---------------------------------------------------------+
| COUNT(student_dormpm.DormCode) * dorm_datapm.DormCharge |
+---------------------------------------------------------+
| 5250.00 |
| 11250.00 |
| 9600.00 |
| 6500.00 |
| 5510.00 |
+---------------------------------------------------------+
But whenever I try to add SUM(), it falls apart and I get an error of Invalid use of group function.
I've tried this SUM(COUNT(student_dormpm.DormCode) * dorm_datapm.DormCharge)

Answer 1

Use sum on entire the first query output where an alias name say t is specified to the first query output (to simulate a new table) and also an alias say total specified to the counter so you can calculate sum(total):
select sum(total) as mysum from
(
SELECT COUNT(student_dormpm.DormCode) * dorm_datapm.DormCharge as total
FROM student_dormpm
JOIN dorm_datapm
USING (DormCode)
GROUP BY DormCode
) t

Answer 2

It looks like dorm_datapm is your dorm table that has one record per DormCode with the associated DormCharge. (In that case a table name dorm ight be more appropriate, though.)
If this is so, then you merely want to sum up the dorm charges:
SELECT DormCode, SUM(dd.DormCharge)
FROM student_dormpm sd
JOIN dorm_datapm dd USING (DormCode)
GROUP BY DormCode
ORDER BY DormCode;
If, however dorm_datapm can have multiple records per DormCode, then you'll have to see, whether above query gets you the results you want or if you want to join a student_dormpm row with only one dorm_datapm row (in which case you would have to extend your join criteria somehow).
If you are only looking for the total, then don't group by DormCode:
SELECT SUM(dd.DormCharge)
FROM student_dormpm sd
JOIN dorm_datapm dd USING (DormCode);