with
sql1 as (select to_char(a) s_name from test_tempa),
R
]F5^5″KFW0n
B0
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*` O3y2eB
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

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