SQL语言查询练习：学生信息、课程信息和成绩

(1) SELECT S#, SNAME FROM S WHERE S# IN (SELECT S# FROM SC WHERE C# = 'MATHS') (2) SELECT S# FROM SC WHERE C# IN ('C1', 'C2') GROUP BY S# HAVING COUNT(DISTINCT C#) = 2 (3) SELECT S#, SNAME, AGE FROM S WHERE AGE BETWEEN 18 AND 20 AND SEX = 'F' (4) SELECT S#, AVG(GRADE) AS AVERAGE_GRADE FROM SC GROUP BY S# HAVING AVG(GRADE) > 80 (5) SELECT SNAME FROM S WHERE NOT EXISTS (SELECT C# FROM C WHERE NOT EXISTS (SELECT S# FROM SC WHERE S.S# = SC.S# AND C.C# = SC.C#))