以下哪条SQL语句可以返回table1中的全部的key：
select tabel1.key from table1 join tabel2 on table1.key=table2.key
select tabel1.key from table1 right outer join tabel2 on table1.key=table2.key
select tabel1.key from table1 left semi join tabel2 on table1.key=table2.key
select tabel1.key from table1 left outer join tabel2 on table1.key=table2.key
