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1、with as语法with as语法 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), 1 -相当于建了个e临时表 2 with e as (select * from scott.emp e where e.empno=7499) 3 select * from e; 4 5 -相当于建了e、d临时表 6 with 7 e as (se
2、lect * from scott.emp), 8 d as (select * from scott.dept) 9 select * from e, d where e.deptno = d.deptno; 其实就是把一大堆重复用到的sql语句放在with as里面,取一个别名,后面的查询就可以用它,这样对于大批量的sql语句起到一个优化的作用,而且清楚明了。 向一张表插入数据的with as用法 1 insert into table2 2 with 3 s1 as (select rownum c1 from dual connect by rownum = 10), 4 s2 as
3、(select rownum c2 from dual connect by rownum = 10) 5 select a.c1, b.c2 from s1 a, s2 b where.; select s1.sid, s2.sid from s1 ,s2需要有关联条件,不然结果会是笛卡尔积。 with as 相当于虚拟视图。 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。 特别
4、对于union all比较有用。因为union all的每个部分可能相同,但是如果每个部分都去执行一遍的话,则成本太高,所以可以使用with as短语,则只要执行一遍即可。如果with as短语所定义的表名被调用两次以上,则优化器会自动将with as短语所获取的数据放入一个temp表里,如果只是被调用一次,则不会。而提示materialize则是强制将with as短语里的数据放入一个全局临时表里。很多查询通过这种方法都可以提高速度。 1 with 2 sql1 as (select to_char(a) s_name from test_tempa), 3 sql2 as (select
5、to_char(b) s_name from test_tempb where not exists 4 (select s_name from sql1 where rownum=1) 5 select * from sql1 6 union all 7 select * from sql2 8 union all 9 select no records from dual 10 where not exists (select s_name from sql1 where rownum=1) and not exists (select s_name from sql2 where rownum=1);