with as語法
–針對乙個別名
with tmp as (select * from tb_name)
–針對多個別名
with
tmp as (select * from tb_name),
tmp2 as (select * from tb_name2),
t*** as (select * from tb_name3),
…
--相當於建了個e臨時表
with e as (select
*from scott.emp e where e.empno=
7499
)select
*frome;
--相當於建了e、d臨時表
with
e as (select
*from
scott.emp),
d as (select
*from
scott.dept)
select
*from e, d where e.deptno = d.deptno;
其實就是把一大堆重複用到的sql語句放在with as裡面,取乙個別名,後面的查詢就可以用它,這樣對於大批量的sql語句起到乙個優化的作用,而且清楚明了。
向一張表插入資料的with as用法
insertinto
table2
with
s1 as (select rownum c1 from dual connect by rownum <=
10),
s2 as (select rownum c2 from dual connect by rownum <=10)
select a.c1, b.c2 from s1 a, s2 b where...;
select s1.sid, s2.sid from s1 ,s2需要有關聯條件,不然結果會是笛卡爾積。
with as 相當於虛擬檢視。
with as短語,也叫做子查詢部分(subquery factoring),可以讓你做很多事情,定義乙個sql片斷,該sql片斷會被整個sql語句所用到。有的時候,是為了讓sql語句的可讀性更高些,也有可能是在union all的不同部分,作為提供資料的部分。
特別對於union all比較有用。因為union all的每個部分可能相同,但是如果每個部分都去執行一遍的話,則成本太高,所以可以使用with as短語,則只要執行一遍即可。如果with as短語所定義的表名被呼叫兩次以上,則優化器會自動將with as短語所獲取的資料放入乙個temp表裡,如果只是被呼叫一次,則不會。而提示materialize則是強制將with as短語裡的資料放入乙個全域性臨時表裡。很多查詢通過這種方法都可以提高速度。
withsql1
as (select to_char(a) s_name from
test_tempa),
sql2
as (select to_char(b) s_name from test_tempb where
notexists (select s_name from sql1 where rownum=1))
select
*from
sql1
union
allselect
*from
sql2
union
allselect
'no records
'from
dual
where
notexists (select s_name from sql1 where rownum=1)
andnot
exists (select s_name from sql2 where rownum=
1);
with as優點
增加了sql的易讀性,如果構造了多個子查詢,結構會更清晰;
更重要的是:「一次分析,多次使用」,這也是為什麼會提供效能的地方,達到了「少讀」的目標
關於oracle with as用法
with as 語法 針對乙個別名 with tmp as select from tb name 針對多個別名 with tmp as select from tb name tmp2 as select from tb name2 t as select from tb name3 1 2 3 ...
關於oracle with as用法
with as 語法 針對乙個別名 with tmp as select from tb name 針對多個別名 with tmp as select from tb name tmp2 as select from tb name2 t as select from tb name3 1 2 3 ...
關於oracle with as用法
with as 語法 針對乙個別名 with tmp as select from tb name 針對多個別名 with tmp as select from tb name tmp2 as select from tb name2 t as select from tb name3 1 2 3 ...