with as关键字的使用

with..as相当于一张中间表，可以简单理解为sql片段

--单个别名
with aliasName as (select ....)
select * from aliasName

-- 多个别名
with
   tmp as (select * from tb_name),
   tmp2 as (select * from tb_name2),
   tmp3 as (select * from tb_name3),
   …

例字

--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;

with as 的这种语法适合和union all搭配使用

with temp1 as (select 'female' sex, 'zhangsan' stu_name from dual),
temp2 as (select 'male' sex, 'lisi' stu_name from dual),
temp3 as (select 'female' sex, 'wangwu' stu_name from dual)

select * from temp1
union all
select * from temp2
union all
select * from temp3