左下角獲取A表加B表的數據，mysql數據庫不支持 full outer join，mysql下可以這麼寫：
select * from tableA A left join tableB B on A.key=B.key
union
select * from tableA A right join tableB B on A.key=B.key
右下角獲取A表獨有的加B表獨有的，mysql數據庫不支持 full outer join，mysql下可以這麼寫：
select * from tableA A left join tableB B on A.key=B.key where B.key is null
union
select * from tableA A right join tableB B on A.key=B.key where A.key is null