而左外连接保留了在操作符左边的未匹配的行，又外连接则保留了在操作符右边的未匹配的行。
内连接是指两端的表都有匹配的行才会出现在结果表中。
--test
create table S(c char(2),a char(2))
insert S select 'c1','a1'
union all select 'c3','a3'
union all select 'c4','a4'
create table T (a char(2),b char(2))
insert T select 'a1','b1'
union all select 'a2','b2'
union all select 'a3','b3'
select S.c,S.a,T.b from S full outer join T on S.a=T.a
select S.c,S.a,T.b from S left outer join T on S.a=T.a
select S.c,S.a,T.b from S right outer join T on S.a=T.a
select S.c,S.a,T.b from S inner join T on S.a=T.a