where, group by, having
where vs having
当一个sql语句中存在where子句，会先执行where，然后执行group by,然后执行having.
一般来说，only use 'having' when you use 'group by'
Always use 'having' with aggregate function.
下面的例子，虽然执行后的结果集一样，但是执行的步骤不同，前者先filter,再group by，后者先group by，然后filter，显然前者更好，而后者浪费了计算资源。
SELECT state, COUNT(*)
FROM Test
WHERE state IN ('CA', 'LA')
GROUP BY state
ORDER BY state
SELECT state, COUNT(*)
FROM Test
GROUP BY state
HAVING state IN ('CA', 'LA')
ORDER BY state