21 . 查询男生、女生人数

SELECT st1.Ssex ,COUNT(Ssex)FROM student AS st1 GROUP BY st1.Ssex

22 . 查询名字中含有「风」字的学生信息

SELECT * FROM student WHERE student.Sname LIKE ‘%风%’

23 . 查询同名同性学生名单，并统计同名人数

SELECT FROM student
WHERE sname IN (
SELECT sname FROM student
GROUP BY sname
HAVING COUNT()>1
);

24 . 查询 1990 年出生的学生名单

SELECT * FROM student WHERE YEAR (student.Sage)= 1990;

25 . 查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列

SELECT sc.CId , course.Cname ,
AVG(sc.score)AS av FROM sc , course
WHERE sc.CId = course.CId
GROUP BY sc.cid
ORDER BY av DESC , CId ASC