SELECT t3.cid, t3.section, (t2.numberA / t3.allStuds) as ‘%A’
FROM (SELECT cid, section, sum(if(grade = ‘A’, 1, 0))numberA
FROM enrollment
GROUP BY cid, section) t2,
(SELECT cid, section, count(*) as allStuds
FROM enrollment
GROUP BY cid, section) t3
WHERE t2.cid = t3.cid AND t2.section = t3.section;