SQL连接

外连接

左外连接：left join 

两表关联，左表全部保留，右表关联不上用null表示。

select * from A left join B on A.id = B.id

右外连接： right join

两表关联，右表全部保留，左表关联不上用null表示。

select * from A right join B on A.id = B.id

内连接：inner join

两表关联，保留两表中交集的记录。

select * from A inner join B on A.id = B.id;

其他连接

1.select * from A left join B on A.id = B.id where B.id is null

2.select * from A right join B on A.id = B.id where A.id is null

3.select * from A full join B on A.id = B.id

4.select * from A full join B on A.id = B.id where A.id is null or B.id is null