(注:student是学生信息表,sc是学生选课表,course是课程信息表)
exists谓词查询
- 查询与其他所有学生年龄均不同的学生学号, 姓名和年龄
(即不存在年龄相同的)
select sno,sname,sage from student s
where not exists (select sage from student t where s.sage=t.sage and s.sno!=t.sno);
- 查询所有选修了1号课程的学生姓名
(即存在选修了1号课程的学生)
select sname from student
where exists (select * from sc where student.sno=sc.sno and cno='1');
- 查询没有选修1号课程的学生姓名
(即不存在选修了1号课程的学生)
select sname from student
where not exists (select * from sc where student.sno=sc.sno and cno='1');
- 查询选修了全部课程的学生姓名
(即不存在没选的课程)
select sname from student
where not exists (select * from course
where not exists (select * from sc where student.sno=sc.sno and sc.cno=course.cno));
- 查询至少选修了 学生95002选修的全部课程的学生的学号
(即不存在95002学生选修的课程没选)
select distinct sno from sc A
where not exists (select * from sc B where B.sno='95002' and not exists
(select * from sc C where A.sno=C.sno and C.cno=B.cno));
- 求没有人选修的课程号cno和cname
select cno,cname from course C
where not exists (select * from sc where sc.cno=C.cno );
相关子查询
- 查询每个学生的所选修的课程成绩最高的成绩信息(sno,cno,grade)
select * from sc A where grade=(select max(grade) from sc where sno=A.sno );
- ☆查询所有学生都选修了的课程的课程号cno
(通过比较当前课程号所选修的人数与SC表中的总共(去重后)人数)
SELECT distinct cno FROM sc
WHERE CNO IN (SELECT CNO FROM SC GROUP BY CNO HAVING COUNT(SNO)
=(SELECT COUNT(DISTINCT SNO) FROM SC));
小结:exists (sql语句 返回结果集为真) not exists (sql语句 不返回结果集为真)
exists : 强调的是是否返回结果集,不要求知道返回什么,返回结果集则条件成立,not exists相反。