(1)统计有学生选修的课程门数。(2)求选修C4课程的学生的平均年龄。(3)求LIU老师所授课程的每门课程的学生平均成绩。(4)统计每门课程的学生选修人员(超过10人的课程才统计)。要求输出课程号和选修人数，查询结果按人数降序排列，若人数相同，按课程号升序排列。(5)检索学号比WANG同学大，而年龄比他小的学生姓名。(6)检索姓名以WANG打头的所有学生的姓名和年龄。
.(1)SELECT COUNT(DISTINCT C#) FROM SC WHERE S# NOT NULL; (2)SELECT AVG(AGE) FROM S,SC WHERE S.S#=SC.S# AND C#=′C4′ (3)SELECT CNAME AVG(GRADE) FROM SC,C WHERE SC.C#=C.C# AND TEACHER=′LIU′ GROUP BY CNAME; (4)SELECT SC.C#,COUNT(*) FROM SC GROUP BY C# HAVING COUNT(*)>10 ORDER BY 2,C# ASC; (5)SELECT X.SNAME FORM S AS X WHERE X.S#>(SELECT * FROM S AS Y WHERE Y.AGE<(SELECT * FROM S AS Z WHERE Z.S#=X.S # AND Z.AGE=Y.AGE AND Z.SNAME=′WANG′)); (6)SELECT SNAME,AGE FROM S WHERE SNAME LIKE′WANG%′; 数据模型如下：
