SQL 中 with as 用法(转)

with

sql1 as (select to_char(a) s_name from test_tempa),

sql2 as (select to_char(b) s_name from test_tempb where not exists (select s_name from sql1 where rownum=1))

select * from sql1ITPUB个人空间2g N*` O3y2e
B Q6}

union all

select * from sql2

union all

select 'no records' from dual

where not exists (select s_name from sql1 where rownum=1)

and not exists (select s_name from sql2 where rownum=1);
再举个简单的例子
with a as (select * from test)
select * from a;

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

这是搜索到的英文文档资料(说得比较全,但是本人英文特菜,还没具体了解到,希望各高手具体谈谈这个with

as 的好处)

About Oracle WITH clause

Starting in Oracle9i release 2 we see an incorporation of the SQL-99 “WITH clause”, a tool for materializing subqueries to save Oracle from having to re-compute them multiple times.

The SQL “WITH clause” is very similar to the use of Global temporary tables (GTT), a technique that is often used to improve query speed for complex subqueries. Here are some important notes about the Oracle “WITH clause”:

• The SQL “WITH clause” only works on Oracle 9i release 2 and beyond.

• Formally, the “WITH clause” is called subquery factoring

• The SQL “WITH clause” is used when a subquery is executed multiple times

• Also useful for recursive queries (SQL-99, but not Oracle SQL)

To keep it simple, the following example only references the aggregations once, where the SQL “WITH clause” is normally used when an aggregation is referenced multiple times in a query.

We can also use the SQL-99 “WITH clause” instead of temporary tables. The Oracle SQL “WITH clause” will compute the aggregation once, give it a name, and allow us to reference it (maybe multiple times), later in the query.

The SQL-99 “WITH clause” is very confusing at first because the SQL statement does not begin with the word SELECT. Instead, we use the “WITH clause” to start our SQL query, defining the aggregations, which can then be named in the main query as if they were
“real” tables:

WITH

subquery_name

AS

(the aggregation SQL statement)

SELECT

(query naming subquery_name);

Retuning to our oversimplified example, let’s replace the temporary tables with the SQL “WITH clause”:

WITH

sum_sales AS

select /*+ materialize */

sum(quantity) all_sales from stores

number_stores AS

select /*+ materialize */

count(*) nbr_stores from stores

sales_by_store AS

select /*+ materialize */

store_name, sum(quantity) store_sales from

store natural join sales

SELECT

store_name

FROM

store,

sum_sales,

number_stores,

sales_by_store

where

store_sales > (all_sales / nbr_stores)

;

Note the use of the Oracle undocumented “materialize” hint in the “WITH clause”. The Oracle materialize hint is used to ensure that the Oracle cost-based optimizer materializes the temporary tables that are created inside the “WITH” clause. This is not necessary
in Oracle10g, but it helps ensure that the tables are only created one time.

It should be noted that the “WITH clause” does not yet fully-functional within Oracle SQL and it does not yet support the use of “WITH clause” replacement for “CONNECT BY” when performing recursive queries.

To see how the “WITH clause” is used in ANSI SQL-99 syntax, here is an excerpt from Jonathan Gennick’s great work “Understanding the WITH Clause” showing the use of the SQL-99 “WITH clause” to traverse a recursive bill-of-materials hierarchy
The SQL-99 “WITH clause” is very confusing at first because the SQL statement does not begin with the word SELECT. Instead, we use the “WITH clause” to start our SQL query, defining the aggregations, which can then be named in the main query
as if they were “real” tables:

WITH

subquery_name

AS

(the aggregation SQL statement)

SELECT

(query naming subquery_name);

Retuning to our oversimplified example, let’s replace the temporary tables with the SQL “WITH” clause”:

另一个例子:

with tempDeptName(deptName) as

(

select

dept_name as deptName

from

bas_dept as dept,tpp_materialmuster as muster

where

dept.DEPT_ID = muster.NEEDUNIT

union all

select

corp_name as deptName

from

bas_corp as corp,tpp_materialmuster as muster

where

corp.corp_id = muster.NEEDUNIT

),

tempProjInfo(projName, projCode) as

(

select

etfprojName as projName,

etfprojCode as projCode

from

tbi_etfproj as etf, tpp_materialMuster as muster

where

etf.etfprojid = muster.projid

union all

select

etmprojName as projName,

etmprojCode as projCode

from

tbi_etmproj as etm, tpp_materialMuster as muster

where

etm.etmprojId = muster.projid

)

select

deptname,

projname,

projcode

from tpp_materialmuster as muster,tempDeptName,tempProjInfo

文章出处：http://www.blogjava.net/sealyu/archive/2009/11/12/302068.html