填空题 设有关系S(S#,SNAME,AGE,SEX)、C(C#,CNAME,TEACHER)和SC(S#,C#,GRADE),请将实现以下功能的SQL语句填写完整: (6)检索选修全部课程的学生姓名(SNAME). SELECT SNAME FROM S WHERE NOT EXISTS (SELECT * FROM C WHERE NOT EXISTS (SELECT * FROM SC WHERE C#= AND S#=S.S#))