SQL多表查询：内连接、外连接(左连接、右连接)、全连接、交叉连接

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内连接  （常用）

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

结果：

Id	Name	ID	Job	Parent_Id
1	张三	1	教师	1
2	李四	2	工程师	2

左连接：保留左表记录，右表被筛掉的为NULL

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

结果：

Id	Name	ID	Job	Parent_Id
1	张三	1	教师	1
2	李四	2	工程师	2
	王五	NULL	NULL	NULL

右连接：保留右表记录，左表被筛掉的为NULL

select a.*,b.* from a right join b on a.id = b.parent_id

结果：

Id	Name	ID	Job	Parent_Id
1	张三	1	教师	1
2	李四	2	工程师	2
NULL	NULL		农民

全连接：

select a.*,b.* from a full join b on a.id = b.parent_id

结果：

Id	Name	ID	Job	Parent_Id
1	张三	1	教师	1
2	李四	2	工程师	2
NULL	NULL		农民
	王五	NULL	NULL	NULL

 交叉连接：表a的每条数据与表b每条数据依次匹配组合（笛卡尔积）a*b

select  a.*,b.*  from a cross join b  on  a.id = b.parent_id

结果：

Id	Name	ID	Job	Parent_Id
1	张三	1	教师	1
1	张三	2	工程师	2
1	张三	3	农民	4
2	李四	1	教师	1
2	李四	2	工程师	2
2	李四	3	农民	4
3	王五	1	教师	1
3	王五	2	工程师	2
3	王五	3	农民	4

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