sqlite - SQLite 中的多个更新触发器

Question

我想做的是管理新生的成绩归属。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，我们希望将该值更改为 NULL。同时，如果插入的元组有等级空值，我们想将其更改为9。

下面的代码有什么问题吗？

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，执行 INSERT 后必须立即对 Student 表执行两条 UPDATE 语句。我们会将空值更新为 9 级，并将等级 > 9 或 > 12 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;

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