设有三个关系: S(S#,SNAME,AGE,SEX) SC(S#,C#,SCORE) C(C#,CNAME,TEACHER) 用关系代数表达式和SQL语句检索WANG同学不学的课程的课程号。
A、πC#(C)-πC#(σSNAME='WANG'(S×SC)) SELECT C# FROM C WHERE NOT EXISTS (SELECT * FROM S, SC WHERE S.S#=SC.S# AND SC.C#=C.C# AND SNAME=’WANG’);
B、πC#(C)-πC#(σSNAME='WANG'(S⋈SC)) SELECT C# FROM C WHERE NOT EXISTS (SELECT * FROM S, SC WHERE S.S#=SC.S# AND SC.C#=C.C# AND SNAME=’WANG’);
C、πC#(SC)-πC#(σSNAME='WANG'(S⋈SC)) SELECT C# FROM SC WHERE EXISTS (SELECT * FROM S, SC WHERE S.S#=SC.S# AND SC.C#=C.C# AND SNAME=’WANG’);
D、πC#(C)-πC#(σSNAME='WANG'(S⋈SC)) SELECT C# FROM SC WHERE NOT EXISTS (SELECT * FROM S, SC WHERE S.S#=SC.S# AND SC.C#=C.C# AND SNAME=’WANG’);