MYSQL内连接，外连接，左连接，右连接

1.select * from tableA A left join tableB B on A.key = B.key

A+B

2.select * from tableA A left join tableB B on A.key = B.key where B.key is NULL

A

3.select * from tableA A inner join tableB B on A.key = B.key

B

4.select * from tableA A right join tableB B on A.key = B.key

B+C

5.select * from tableA A right join tableB B on A.key = B.key where A.key is NULL

C

6.select * from tableA A outer join tableB B on A.key = B.key

A+B+C

7.select * from tableA A outer join tableB B on A.key = B.key where A.Key is NULL B.key is NULL

A+B