with as 用法

WITH AS语句，也称为子查询一部分（subquery factoring），能够 使你做许多事儿，界定一个SQL片段，该SQL片段会被全部SQL语句常用到。有的情况下，是为了更好地让SQL语句的易读性更高些，也是有很有可能是在UNION ALL的不一样一部分，做为给出的数据的一部分。

with as 用法

–对于一个别称

with tmp as (select * from tb_name)

–对于好几个别称

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;