SQL JOINS
①左外连接
select field from TableA A left join TableB B on A.key = B.key
②左外连接，去掉右侧键值
select field from TableA A left join TableB B on A.key = B.key
where B.key IS NULL
③内连接，
select field from TableA A inner join Table B on A.key = B.key
④右外连接
select field from TableA A right join Table B on A.key = B.key
⑤右外连接，去掉左侧键值
select field from TableA A right join TableB B on A.key = B.key
where A.key IS NULL
⑥全连接
select field from TableA A full outer join TableB B on A.key = B.key
⑦全连接，去掉左右键值
select field from TableA A full outer join TableB B on A.key = B.key