with as 用法

    WITH AS短语，也叫做子查询部分（subquery factoring），可以让你做很多事情，定义一个SQL片断，该SQL片断会被整个SQL语句所用到。有的时候，是为了让SQL语句的可读性更高些，也有可能是在UNION ALL的不同部分，作为提供数据的部分。

    

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;