SSD7——数据库实验（五）

数据库实验五

还是用实验一建立的那三个表进行操作
相关知识请看这个博客，尤其是游标说的很清楚
（1）设置一个触发器，该触发器仅允许“dbo”用户可以删除学生表内数据，否则出错
说明：deleted表用于存储INSERT和UPDATE语句所影响的行的副本

CREATE TRIGGER dbo_delete
ON S
FOR DELETE
AS
	--这里指定了条件
	IF EXISTS (SELECT * FROM deleted)
		BEGIN
			IF USER != 'dbo'
		END
	DELETE FROM S WHERE S.Sno = '1'

（2）针对学生表写一个DELETE触发器

CREATE TRIGGER S_delete
ON S
FOR DELETE
AS
	PRINT('DELETE')
DELETE FROM S WHERE S.Sno = '1'

（3）针对学生表写一个UPDATE触发器

CREATE TRIGGER S_update
ON S
FOR UPDATE
AS
	PRINT(UPDATE)
UPDATE S SET S.sdept = 'CS' WHERE S.Sno = '1'

（4）统计学生的平均成绩，输出低于平均分的成绩（使用游标）
这篇博客比较详细

DECLARE lowavg CURSOR
FOR
	SELECT SC.grade
	FROM SC
	WHERE SC.grade < (SELECT avg(SC.grade) FROM SC)

OPEN lowavg
FETCH NEXT
FROM S
INTO @grade

WHILE @@fetch_status=0
    BEGIN
		PRINT(@grade)
        FETCH NEXT FROM lowavg INTO @grade
    END
CLOSE lowavg
DEALLOCATE lowavg

如图：

实验五结束。