Question

我想要做的是管理新学生的成绩归属。如果插入的包含学生ID和成绩的元组的成绩值小于9或大于12，我们希望将值更改为NULL。同时，如果插入的元组具有等级null值，我们希望将其更改为9。

以下代码有什么问题？

CREATE TRIGGER R1
AFTER INSERT ON Students
FOR EACH ROW 
BEGIN

  UPDATE Students
     SET grade = NULL 
   WHERE grade < 9 OR grade > 12;

  UPDATE Students
     SET grade = 9 
   WHERE grade = NULL;

END;

请注意，在执行INSERT之后，必须在Student表上执行两个UPDATE语句。我们将空值更新为9级和等级＆gt; 9或＆gt; 12要更新为NULL

Answer 1

您应该将第二个条件从= NULL替换为IS NULL：

UPDATE Students
     SET grade = 9 
   WHERE grade IS NULL;

使用= NULL和IS NULL证明：http://sqlfiddle.com/#!5/a51de/1

其他解决方案：

http://sqlfiddle.com/#!5/78288/1

CREATE TRIGGER TStudents3
AFTER INSERT ON Students3
FOR EACH ROW
/* we check update conditions here */
WHEN  NEW.grade IS NULL
  OR  NEW.grade < 9
  OR  NEW.grade > 12
BEGIN

  UPDATE Students3
     SET grade =
       CASE
         WHEN grade IS NULL THEN 9
         /* actually you could drop this 2 lines, because a missing ELSE returns NULL: */
         WHEN grade < 9 OR grade > 12 THEN NULL
         ELSE grade
       END
   /* We just want to update the freshly inserted row, not all row! */
   WHERE  id = NEW.id;

END

Answer 2

另一种方法是在where子句中移动条件：

CREATE TRIGGER tr1 AFTER INSERT ON Student
BEGIN
     UPDATE Student SET grade = null WHERE id = NEW.id and (new.grade<9 or new.grade>12);
     UPDATE Student SET grade = 9 WHERE id = NEW.id and new.grade is null;
END;

SQLite中的多个更新触发器

2 个答案: