集合查询

集合查询
例题48
查询计算机专业的学生和年龄不大于19岁的学生   union
select * from tab_Student
where Sdept='CS'
union
select * from tab_Student
where Sage<=19
例题49
查询选修了课程1或者选修了课程2的学生信息    union
select * from tab_Student
where exists (select * from tab_SC where Sno=tab_Student.Sno and Cno=1)
union
select * from tab_Student
where exists (select * from tab_SC where Sno=tab_Student.Sno and Cno=2)
例题50
查询计算机系的同学和年龄不大于19的学生的交集  intersect
select * from tab_Student
where Sdept='CS'
intersect
select * from tab_Student
where Sage<=19
实际上上面那句等同于
select * from tab_Student
where Sdept='CS' and Sage<=19