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语句检索至少选修两门课的学生学号。
A、π1(σ1=4 ∧ 2≠5(SC⋈SC)) SELECT DISTINCT X.S# FROM SC AS X, SC AS Y WHERE X.S#=Y.S# AND X.C#!=Y.C#;
B、π1(σ1=4 ∨ 2≠5(SC×SC)) SELECT DISTINCT X.S# FROM SC AS X, SC AS Y WHERE X.S#=Y.S# AND X.C#!=Y.C#;
C、π1(σ1=4 ∨ 2≠5(SC×SC)) SELECT DISTINCT X.S# FROM SC , SC WHERE X.S#=Y.S# AND X.C#!=Y.C#;
D、π1(σ1=4 ∧ 2≠5(SC×SC)) SELECT DISTINCT X.S# FROM SC AS X, SC AS Y WHERE X.S#=Y.S# AND X.C#!=Y.C#;