with as 用法

WITH 6日活 AS短语，也叫做子查询部分到物（subquery factoring），可以侣结让你做很多事情，定义一个洲节SQL片断，该SQL片断会被整个SQL语句视什所用到。有海姑的时候，是为了让S堂 QL语句的可读性更高些，也有可能是图明在UNION ALL的不同部分，作为提供数据捧腹的部分。

with as 用法

–针对一个别名

with tmp as (select * from tb_name)

–针对多个别名

with

tmp as (select * from tb_name),

tmp2 as (select * from tb_name2),

tmp3 as (select * from tb_name3),

…

–相当于建了个e临时表

with e as (select * from scott.emp e where e.empno=7499)

select * from e;

–相当于建了e、d临时表

with

e as (select * from scott.emp),

d as (select * from scott.dept)

select * from e, d where e.deptno = d.deptno;