概述

WITH AS短语，也叫做子查询部分（subquery factoring），可以做很多事情，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。

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01

with as 语法

单个语法：

with tempName as (select ....)
select ...

多个语法：

with tempName1 as (select ....)，
tempName2 as (select ....)，
tempName3 as (select ....) ...
select ...

With查询语句不是以select开始的，而是以“WITH”关键字开头

可认为在真正进行查询之前预先构造了一个临时表TT，之后便可多次使用它做进一步的分析和处理。

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02

WITH AS实例

例：从1-19中得到11-14。一般的sql如下：

select * from ( --模拟生一个20行的数据
SELECT LEVEL AS lv FROM DUAL CONNECT BY LEVEL < 20
) tt WHERE tt.lv > 10 AND tt.lv < 15

 

 

使用With as 的SQL为：

with TT as( SELECT LEVEL AS lv FROM DUAL CONNECT BY LEVEL < 20 ) select lv from TT WHERE lv > 10 AND lv < 15;

 

多个临时表实例：

WITH
T3 AS
(SELECT T1.ID, T1.CODE1, T2.DESCRIPTION
FROM TB_DATA T1, TB_CODE T2
WHERE T1.CODE1 = T2.CODE
),
T4 AS
(SELECT T1.ID, T1.CODE2, T2.DESCRIPTION
FROM TB_DATA T1, TB_CODE T2
WHERE T1.CODE2 = T2.CODE
)
SELECT T3.ID, T3.DESCRIPTION, T4.DESCRIPTION
FROM T3, T4
WHERE T3.ID = T4.ID
ORDER BY ID;

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03

WITH Clause方法的优点

增加了SQL的易读性，如果构造了多个子查询，结构会更清晰；更重要的是：“一次分析，多次使用”，这也是为什么会提供性能的地方，达到了“少读”的目标。

上面第一种使用子查询的方法表被扫描了两次，而使用WITH Clause方法，表仅被扫描一次。这样可以大大的提高数据分析和查询的效率。

另外，观察WITH Clause方法执行计划，其中“SYS_TEMP_XXXX”便是在运行过程中构造的中间统计结果临时表。

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04

WITH AS 与增删改查结合用法

4.1与select查询语句结合使用

查询同一个单据编号对应的借款单和核销单中，借款金额不相等的单据

with verificationInfo as
 (select ment.fnumber from shenzhenjm.t_finance_expenseremburseitem t
 left join shenzhenjm.t_finance_expenserembursement ment
 on ment.fid = t.fkrembursementid
 where 1 = 1
 group by ment.fnumber),
 
loanInfo as
 (select ment.fnumber
 from shenzhenjm.t_finance_expenseremburseitem2 t
 left join shenzhenjm.t_finance_expenserembursement ment
 on ment.fid = t.fkrembursementid
 where 1 = 1
 group by ment.fnumber)
 
select *
 from verificationInfo v, loanInfo l
 where l.fnumber = v.fnumber;

4.2与insert结合使用

如下的with as语句，不能放在insert前，而是放在紧接着要调用的地方前

insert into T_finance_ExpenseRemburseItem2
 (FID,
 FKREMBURSEMENTID,
 FAMOUNT,
 FKCREATEBYID,
 FCREATETIME,
 FKCUID,
 FKCOSTTYPEID,
 FCOSTTYPENAME)
 
 with verificationInfo as
 (select ment.fnumber,
 sum(t.famount) vLoanSum,
 ment.fnumber "单据编号",
 sum(t.famount) "核销单中借款总额"
 from shenzhenjm.t_finance_expenseremburseitem t
 left join shenzhenjm.t_finance_expenserembursement ment
 on ment.fid = t.fkrembursementid
 where 1 = 1
 group by ment.fnumber),
 
 loanInfo as
 (select ment.fnumber,
 sum(t.famount) loanSum,
 ment.fnumber "单据编号",
 sum(t.famount) "借款单中借款总额"
 from shenzhenjm.t_finance_expenseremburseitem2 t
 left join shenzhenjm.t_finance_expenserembursement ment
 on ment.fid = t.fkrembursementid
 where 1 = 1
 group by ment.fnumber)
 
 select sys_guid(),
 ment.fid,
 t.famount,
 ment.fkcreatebyid,
 ment.fcreatetime,
 ment.fkcuid,
 t.fkcosttypeid,
 t.fcosttypename
 from T_finance_ExpenseRemburseItem t
 left join t_finance_expenserembursement ment
 on ment.fid = t.fkrembursementid
 where 1 = 1
 and exists (select *
 from verificationInfo v, loanInfo l
 where l.fnumber = v.fnumber
 and l.loanSum != v.vLoanSum
 and v.fnumber = ment.fnumber);

4.3 与delete删除结合使用

delete from t_finance_expenseremburseitem2 item2
 where exists(with temp as (select t.fnumber,
 sum(item1.famount) vloanSum,
 sum(item1.frealityamount) vSum,
 sum(item2.famount) loanSum
 from t_finance_expenserembursement t
 left join t_finance_expenseremburseitem item1
 on item1.fkrembursementid = t.fid
 left join t_finance_expenseremburseitem2 item2
 on item2.fkrembursementid = t.fid
 where 1 = 1
 and t.frembursementtype = 'LOAN_REPORT'
 and to_char(t.fcreatetime, 'yyyy') > '2017'
 group by t.fnumber
 order by t.fnumber asc)
 
 select 1
 from temp t
 left join t_finance_expenserembursement ment
 on t.fnumber = ment.fnumber
 left join t_finance_expenseremburseitem2 item
 on item.fkrembursementid = ment.fid
 where t.vloanSum != t.loanSum
 and item.fid = item2.fid);

4.4与update结合使用

update dest b
 set b.NAME =
 (with t as (select * from temp)
 select a.NAME from temp a where a.ID = b.ID)

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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。后面会分享更多devops和DBA方面的内容，感兴趣的朋友可以关注一下