设有学生-课程关系数据库,它由三个关系组成。它们的模式是:学生S(学号S#,姓名SN,所在系SD,年龄SA)、
select distinct Sn
from S,C,SC
where S.S#=SC.S#
and C.C#=SC.C#
and Cn in(select Cn from S,C,SC
where S.S#=SC.S#
and C.C#=SC.C#
and S.S#='s5')
select distinct Sn
from S,C,SC
where S.S#=SC.S#
and C.C#=SC.C#
and Cn in(select Cn from S,C,SC
where S.S#=SC.S#
and C.C#=SC.C#
and S.S#='s5')