教学数据库关系模式为: S(S#,SNAME,AGE,SEX) SC(S#,C#,SCORE) C(C#,CNAME,TEACHER)现对基本表SC,建立一个视图: CREATE VIEW S_GRADE(S#,C_NUM,AVG_GRADE) AS SELECT S#,COUNT(C#),AVG(GRADE) FROM SC GROUP BY S#; 试判断查询操作是否允许执行。SELECT S#,AVG_GRADE FROM S_GRADEWHERE C_NUM >(SELECT C_NUMFROM S_GRADEWHERE S#='S4');