sql的运行顺序

(1) from
(2) join
(3) on
(4) where
(5) group by(开始使用select中的别名，后面的语句中都可以使用)
(6) avg.sum...
(7) having
(8) select
(9)distinct
(10) distribute by /cluster by
(11) sort by
(12) order by
(13) limit
(14) union /union all