下列关于触发器的定义中，正确的是（）。
A. DELIMITER $$ CREATE TRIGGER tr_stu AFTER INSERT ON tb_student FOR EACH ROW BEGIN DELETE FROM tb_sc WHERE sno=OLD.sno; END$$
B. DELIMITER $$ CREATE TRIGGER tr_stu AFTER DELETE ON tb_student FOR EACH ROW BEGIN DELETE FROM tb_sc WHERE sno=OLD.sno; END$$
C. DELIMITER $$ CREATE TRIGGER tr_stu BEFORE INSERT（sno） ON tb_student FOR EACH ROW BEGIN DELETE FROM tb_sc WHERE sno=NEW.sno; END$$
DELIMITER $$ CREATE TRIGGER tr_stu AFTER DELETE ON tb_student FOR EACH ROW BEGIN DELETE FROM tb_sc WHERE sno=NEW.sno; END$$
设smajor是student表中的一个字段，以下能够正确判断smajor字段是否为空值的表达式是（）。
A. smajor = NULL
B. smajor IS NULL
C. smajor=0
D. smajor=’’
设有一个成绩表Student_JAVA(id，name，grade)，现需要查询成绩grade第二名的同学信息（假设所有同学的成绩各不相同），正确的SQL语句应该是（）。
A. SELECT * FROM Student_JAVA ORDER BY grade limit 1,1;
B. SELECT * FROM Student_JAVA ORDER BY grade DESC limit 1,1;
C. SELECT * FROM Student_JAVA ORDER limit 1,1;
D. SELECT * FROM Student_JAVA ORDER BY grade DESC limit 0,1;
设有学生表student，包含的属性有学号sno、学生姓名sname、性别sex、年龄age、所在专业smajor。基于student表建立视图，其中可更新的视图是（）。
A. CREATE VIEW V1 AS SELECT sno,sname FROM student;
B. CREATE VIEW V2(major) AS SELECT DISTINCT(smajor) FROM student;
CREATE VIEW V3(major,scount) AS SELECT smajor,COUNT(*) FROM student GROUP BY smajor;
D. CREATE VIEW V4(sname,sage) AS SELECT sname,age+5 FROM student where sno='101';
微信支付 
支付宝支付