有关系模式Teacher(Tno,Tname,Tage,Tset),要求在更新教师年龄时,如果新年龄比旧年龄低则 用旧年龄代替。该完整性约束该用什么机制实现_________?
A.完整性约束命名子句
B.参照完整性
C.断言
D.触发器
- · 有4位网友选择 C,占比44.44%
- · 有3位网友选择 D,占比33.33%
- · 有2位网友选择 B,占比22.22%
A.完整性约束命名子句
B.参照完整性
C.断言
D.触发器
A、πS#,C#(SC)÷πC#(σTNAME='LIU'(SC)) SELECT DISTINCT S# FROM SC AS X WHERE NOT EXISTS (SELECT * FROM C WHERE TNAME=’LIU’ AND NOT EXISTS (SELECT * FROM SC AS Y WHERE Y.S#=X.S# AND Y.C#=C.C#));
B、πS#,C#(SC)÷πC#(σTNAME='LIU'(C)) SELECT DISTINCT S# FROM SC AS X WHERE NOT EXISTS (SELECT * FROM C WHERE TNAME=’LIU’ AND NOT EXISTS (SELECT * FROM SC AS Y WHERE Y.S#=X.S# AND Y.C#=C.C#));
C、πS#,C#(SC)÷πC#(σTNAME='LIU'(C)) SELECT DISTINCT S# FROM SC AS X WHERE NOT EXISTS (SELECT * FROM C WHERE TNAME=’LIU’ AND EXISTS (SELECT * FROM SC AS Y WHERE Y.S#=X.S# AND Y.C#=C.C#));
D、πS#,C#(SC)÷πC#(σTNAME='LIU'(C)) SELECT DISTINCT S# FROM SC WHERE NOT EXISTS (SELECT * FROM C WHERE TNAME=’LIU’ AND EXISTS (SELECT * FROM SC AS Y WHERE Y.S#=X.S# AND Y.C#=C.C#));
A、{ <t[1],t[4]> | tÎC Ù $ (uÎS) $ (vÎSC)(u[1]=‘S3’ Ùu[1]=v[1] Ùt[1]=v[2])
B、
C、
D、
B.tno、tnam
C.*
D.tno,tname,dept
A、
B、
C、
D、
学生关系S(SNO,SNAME,AGE,SEX)(分别代表学生的学号、姓名、年龄、性别)
学习关系SC(SNO,CNO,GRADE)(分别代表学生的学号、课程编号、成绩)
课程关系C(CNO,CNAME,TEACHER)(分别代表课程的编号、课程名、任课教师)
“从学习关系SC中检索无成绩的学生学号”的SQL语句是
A.SELECT sno FROM sc WHERE grade=NULL
B.SELECT sno FROM sc WHERE grade IS
C.SELECT sno FROM sc WHERE grade IS NULL
D.SELECT sno FROM sc WHERE grade=0
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