为表ReaderType的LimitNum字段添加检查约束，要求教师类型中该字段的值+3，正确的语句是 A: ALTER TABLE ReaderTypeADD CONSTRAINT CK_ReaderType_LimitNum check (LimitNum=LimitNum+3) B: ALTER TABLE ReaderTypeADD CONSTRAINT check (LimitNum=LimitNum+3) WHERE Typename='教师' C: ALTER TABLE ReaderTypeADD CONSTRAINT CK_ReaderType_LimitNum (LimitNum=LimitNum+3) WHERE Typename='教师' D: ALTER TABLE ReaderTypeADD CONSTRAINT CK_ReaderType_LimitNum check (LimitNum=LimitNum+3) WHERE Typename='教师'

为表ReaderType的LimitNum字段添加检查约束，要求教该字段的值<3，正确的语句是 A: ALTER TABLE ReaderTypeADD CONSTRAINT CK_ReaderType_LimitNum check (LimitNum=3) B: ALTER TABLE ReaderTypeADD CONSTRAINT check (LimitNum<3) C: ALTER TABLE ReaderTypeADD CONSTRAINT CK_ReaderType_LimitNum (LimitNum<3) D: ALTER TABLE ReaderTypeADD CONSTRAINT CK_ReaderType_LimitNum check (LimitNum<3)