数据库withas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;