1、运行如下语句:
declare @t table( t1 int)
declare @tt table(t1 int ,t2 varchar(10))
insert into @t values(1)
insert into @t values(2)
insert into @t values(3)
insert into @t values(4)
insert into @tt values(1,'ab')
insert into @tt values(1,'acd')
insert into @tt values(2,'ab')
insert into @tt values(3,'a')
insert into @tt values(2,'a')
set statistics profile on
select *
from @t x
left outer join @tt y
on x.t1 = y.t1
where x.t1 = 1 and y.t2 like '%a%'
结果:
select * from @t x left outer join @tt y on x.t1 = y.t1 where x.t1 = 1 and y.t2 like '%a%'
|--Nested Loops(Inner Join)
|--Table Scan(OBJECT:(@tt AS [y]), WHERE:(@tt.[t1] as [y].[t1]=(1) AND @tt.[t2] as [y].[t2] like '%a%'))
|--Table Scan(OBJECT:(@t AS [x]), WHERE:(@t.[t1] as [x].[t1]=(1)))
2、把上面语句中的left outer join 改为inner join后
结果:
select * from @t x inner join @tt y on x.t1 = y.t1 where x.t1 = 1 and y.t2 like '%a%'
|--Nested Loops(Inner Join)
|--Table Scan(OBJECT:(@tt AS [y]), WHERE:(@tt.[t1] as [y].[t1]=(1) AND @tt.[t2] as [y].[t2] like '%a%'))
|--Table Scan(OBJECT:(@t AS [x]), WHERE:(@t.[t1] as [x].[t1]=(1)))
3、如果过滤条件放到 on 子句里面,把上面的SQL改为:
select *
from @t x
left outer join @tt y
on x.t1 = y.t1 and y.t2 like '%a%'
where x.t1 = 1
产生的执行计划:
select * from @t x left outer join @tt y on x.t1 = y.t1 and y.t2 like '%a%' where x.t1 = 1
|--Nested Loops(Left Outer Join)
|--Table Scan(OBJECT:(@t AS [x]), WHERE:(@t.[t1] as [x].[t1]=(1)))
|--Table Scan(OBJECT:(@tt AS [y]), WHERE:(@tt.[t1] as [y].[t1]=(1) AND @tt.[t2] as [y].[t2] like '%a%'))
其实对比上面的1、2、3,会发现在Table Scan里都是一样的 :
|--Table Scan(OBJECT:(@t AS [x]), WHERE:(@t.[t1] as [x].[t1]=(1)))
|--Table Scan(OBJECT:(@tt AS [y]), WHERE:(@tt.[t1] as [y].[t1]=(1) AND @tt.[t2] as [y].[t2] like '%a%'))
而且在两个表关联之前,在Table Scan中已经用where子句过滤条件进行过滤,如果用到on子句,就会转化成where子句条件,
只是对不同的情况,有不同的处理。
用left outer join做关联时:
一、当把右边表的字段过滤条件写在where中时(不用转化),在执行计划中是:Nested Loops(inner join)。
这里SQL Server 做了优化(inner join的效率优于left outer join)。
之所以可以做这样的优化,是由于把右边表的字段过滤条件直接放到where子句中,对联接产生的结果集合中左边表有,而右边表没有的记录,在联接结果集中此字段的值必定为NULL,那么通过这个字段的过滤,必定把NULL的记录都过滤掉了,此时效果等同于inner join,所以把外联接(left outer join)转化成内联接(inner join)。
二、当把右边表的字段过滤条件写在on中时(把on转化成where子句),执行计划中是:Nested Loops(left outer join),无法优化。
因为由left outer join产生的结果集与inner join产生的结果集是不同的,无法优化。