MySQL 联表速查

假设有如下两张表:

A:

id age
1 11
2 12
3 13
4 14

B:

id name
2 aaa
3 bbb
4 ccc
5 ddd

内连接 (inner join)

只有一种应用场景,inner join或join,等同于inner join

select a.*,b.* from a inner join(join) b on a.id = b.id

结果如下:

id age id name
2 12 2 aaa
3 13 3 bbb
4 14 4 ccc

外连接

1.left join 或者 left outer join

select a.*,b.* from a left join b on a.id = b.id

结果如下:

id age id name
1 11 null null
2 12 2 aaa
3 13 3 bbb
4 14 4 ccc

2.left join + where b.column is null

select a.*,b.* from a left join b on a.id = b.id where b.id is null

结果如下:

id age id name
1 11 null null

3.right join 或者right out join

select a.*,b.* from a right join b on a.id = b.id
id age id name
2 12 2 aaa
3 13 3 bbb
4 14 4 ccc
null null 5 ddd

4.left join + where b.column is null

select a.*,b.* from a right join b on a.id = b.id where a.id is null
id age id name
null null 5l ddd

5.left join   union right join (可用full join 但mysql不支持)

select a.id aid,a.age,b.id bid,b.name from  a

left join  b

on a.id = b.id

union

select a.id aid,a.age,b.id bid,b.name from  a

right join  b

on a.id = b.id
aid age bid name
1 11 null null
2 12 2 aaa
3 13 3 bbb
4 14 4 ccc
null null 5 ddd

6.(left join + is null) union (right join + is null)

select a.id aid,a.age,b.id bid,b.name from  a

left join  b

on a.id = b.id

where b.id is null

union

select a.id aid,a.age,b.id bid,b.name from  a

right join  b

on a.id = b.id

where a.id is null
aid age bid name
1 11 null null
null null 5 ddd

交叉连接 (cross join)

实际应用中还有这样一种情形,想得到A,B记录的排列组合,即笛卡儿积,这个就不好用集合和元素来表示了。需要用到cross join。

笛卡尔积:笛卡尔乘积是指在数学中,两个集合X和Y的笛卡尓积(Cartesian product),又称直积,表示为X × Y,第一个对象是X的成员而第二个对象是Y的所有可能有序对的其中一个成员。

select a.id aid,a.age,b.id bid,b.name from tablea a
cross join tableb b
aid age bid name
1 11 2 aaa
2 12 2 aaa
3 13 2 aaa
4 14 2 aaa
1 11 3 bbb
2 12 3 bbb
3 13 3 bbb
4 14 3 bbb
1 11 4 ccc
2 12 4 ccc
3 13 4 ccc
4 14 4 ccc
1 11 5 ddd
2 12 5 ddd
3 13 5 ddd
4 14 5 ddd

还可以指定where条件

select a.id aid,a.age,b.id bid,b.name from  a
cross join  b
where a.id = b.id
aid age bid name
2 12 2 aaa
3 13 3 bbb
4 14 4 ccc

注意事项:

  1. 一般cross join后面加上where条件,但是用cross join+on也是被解释为cross join+where;

  2. 一般内连接都需要加上on限定条件,如内连接;如果不加会被解释为交叉连接;

  3. 如果连接表格使用的是逗号,会被解释为交叉连接;

本文摘自:https://blog.csdn.net/Jintao_Ma/article/de...

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