MySQL 触发器和 SUM()

Question

我有两张桌子学生和家庭。在学生中，我有列 st_income 和 total_income。在家里，我有收入。 Total_income 是学生的 st_income 和 family.id_student=student.id_student 的家庭收入的总和。

我想通过触发器更新 total_income，我做了这个

CREATE TRIGGER family_income_update AFTER UPDATE ON `family` 
FOR EACH ROW UPDATE student SET total_income= ((SELECT SUM(income) 
FROM family WHERE family.id_student=student.id_student)+
(SELECT st_income FROM student WHERE student.id_student=NEW.id_student)) 
WHERE student.id_student=NEW.id_student 

MySQL 接受了这个触发器，但是当我想更新表族中的归档收入时，我得到了这样的沟通：

1093 - 您不能在 FROM 子句中指定目标表 'student' 进行更新

我不知道如何解决这个问题。

更新

我试过这个：

CREATE TRIGGER family_income_update AFTER UPDATE ON `family` 
FOR EACH ROW 

SET @familyIncome = SELECT SUM(income) FROM family WHERE family.id_student=student.id_student
SET @studentIncome= SELECT st_income FROM student WHERE student.id_student=NEW.id_student
SET @totalIncome=@familyIncome+@studentIncome

UPDATE student SET total_income=@totalIncome WHERE student.id_student=NEW.id_student

但我得到了这个答案：

1064 - 您的 SQL 语法有错误；检查与您的 MySQL 服务器版本相对应的手册，以在第 4 行的“SELECT SUM(income) FROM family WHERE family.id_student=student.id_student”附近使用正确的语法

更新

我尝试添加到 st_income 的 total_income 值。两列都在同一个表中。我试过这个：

CREATE TRIGGER `st_income_update` AFTER UPDATE ON `student`
 FOR EACH ROW BEGIN
  UPDATE student
    SET total_income = total_income + (NEW.st_income - OLD.st_income)
    WHERE student.id_student = NEW.id_student;
END

但它会导致错误。

Answer 1

这就是所谓的“变异表”问题。归结为，不允许行触发器访问同一个表中的其他行，因为不能保证行将按特定顺序更新等等。

首先，当您可以在需要时轻松计算时，您真的不应该尝试存储 total_income。也就是说，我认为你可以通过做类似的事情来做你想做的事

CREATE TRIGGER family_income_update
  AFTER UPDATE ON family
  FOR EACH ROW 
BEGIN
  UPDATE student
    SET total_income = total_income + (NEW.income - OLD.income)
    WHERE student.id_student = NEW.id_student;
END;

这个想法是修改 student.total_income 相对于其先前的值，因为您只能引用 family.income 相对于其先前的值。

Answer 2

尝试在选择中重命名学生表的别名

CREATE TRIGGER family_income_update AFTER UPDATE ON `family` 
FOR EACH ROW UPDATE student SET total_income= ((SELECT SUM(income) 
FROM family WHERE family.id_student=student.id_student)+
(SELECT b.st_income FROM student as b WHERE  b.id_student=NEW.id_student)) 
WHERE student.id_student=NEW.id_student 

Answer 3

如果您在触发器中使用变量来计算总收入，然后在更新查询中使用该变量？

类似的东西

CREATE TRIGGER family_income_update AFTER UPDATE ON `family` 
FOR EACH ROW 

SET @totalIncome=(SELECT (st.st_income + ifnull(fam.income, 0)) as     total_income
                  FROM student st left join (
                            SELECT id_student, SUM(income) as income
                            FROM family
                            GROUP BY id_student
                        ) fam on fam.id_student = st.id_student
                  WHERE st.id_student = NEW.id_student);

UPDATE student 
SET total_income = @totalIncome 
WHERE student.id_student=NEW.id_student;