27.参考答案及评分要点： (1)CREATE TABLE S( SNo INT PRIMARY KEY, SN CHAR(30), SA INT, SD CHAR(20) }； (3分) (2)CREATE TABLE C( CNo INT PRIMARY KEY, CN CHAR(20), T CHAR(20) }； (3分) (3)CREATE TABLE SC( SNo INT, CNo INT, G INT, PRIMARY KEY(SNo,CNo), FOREIGN KEY(SNo)REFERENCES S(SNo), FOREIGN KEY(CNo)REFERENCES C(CNo) }； (4分) 28.参考答案及评分要点： SELECT SNo,AVG(G) FROM SC 112427. 参考 及评 (1) CREATE TABLE S{ SNo INT PRIMARY KEY , SN CHAR(30) , SA INT, SD CHAR(20) (3 (2) CREATE TABLE C{ CNo INT PRIMARY KEY , CN CHAR(20) , T CHAR(20) (3 (3)CREATE TABLE SC{ SNo INT, CNo INT, G INT, PRIMARY KEY(SNo ,CNo) , FOREIGN KEY(SNo) REFERENCES S(SNo) , FOREIGN KEY(CNo) REFERENCES C(CNo) (4 28. 案及 SELECT SNo ,AVG(G) FROM SC 1124