MySQL七种join理论

1. 内连接

select * from A inner join B where A.key=B.key;

2. 左连接

select * from A left join B on A.key=B.key where B.key is null;

3. 右连接

select * from A right join B on A.key=B.key where A.key is null;

4. 左外连接

select * from A left join B on A.key=B.key;

5. 右外连接

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

6. 全外连接

select * from A left join B where A.key=B.key union select * from A right join B where A.key=B.key;

7. 两表独有的数据集

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

union 

select * from A right join B on A.key=B.key where A.key is null;