// 下面這句是可以的
SELECT COUNT(*) FROM A
WHERE A.a1>0
GROUP BY a2；

//但是下面就不行
SELECT COUNT(*) FROM A
WHERE A.a1>0 and count(*)>1
GROUP BY a2；

//必須要改為having
SELECT COUNT(*) FROM A
WHERE A.a1>0
GROUP BY a2
HAVING COUNT(*)>1