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