left join,inner join,right join,cross join,full join的区别
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left join,inner join,right join,cross join,full join的区别
1,总论:LEFT JOIN返回”first_table”中所有的行尽管在” second_table”中没有相匹配的数据。
RIGHT JOIN返回”second_table”中所有的行尽管在”first_table”中没有相匹配的数据。
INNER JOIN返回的结果集是两个表中所有相匹配的数据。
没听明白?请继续看下文分解。
2,分解:
还是用一个例子来的痛快些。。。
declare @a table(a int,b int)
declare @b table(a int,b int)
insert @a values(1,1)
insert @a values(2,2)
insert @b values(1,1)
insert @b values(3,3)
--左:
select * from @a Aa left join @b Bb on Aa.a=Bb.a
--右:
select * from @a Aa right join @b Bb on Aa.a=Bb.a
--内
select * from @a Aa join @b Bb on Aa.a=Bb.a
--外:
select * from @a Aa full join @b Bb on Aa.a=Bb.a
--完全
select * from @a,@b
cross join 是笛卡儿乘积就是一张表的行数乘以另一张表的行数
left join 第一张表的连接列在第二张表中没有匹配是,第二张表中的值返回null
right join 第二张表的连接列在第一张表中没有匹配是,第一张表中的值返回null
full join 返回两张表中的行 left join+right join
inner join 只返回两张表连接列的匹配项
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