Aggregates & GROUP BY

Aggregate functions squash many rows into one answer:

Function	Answers
COUNT(*)	how many rows?
SUM(field)	total
AVG(field)	average
MAX(field) / MIN(field)	highest / lowest

Give the answer column a decent heading with AS:

SELECT COUNT(*) AS Total
FROM tblLearners
WHERE Grade = 12

GROUP BY splits the rows into groups first and aggregates each group — one result row per group:

SELECT Grade, AVG(ITMark) AS AvgMark
FROM tblLearners
GROUP BY Grade

Golden rule: every field in the SELECT that is not inside an aggregate must appear in the GROUP BY. (Break the rule here and read the error — it is the same one Access gives.)