如何在触发器中将列增加1？

Question

我需要帮助编写MySQL触发器。

假设您有一个包含以下表格的学生数据库：

ENROLLMENT(SSN, CLASS_NO, GRADE)
CLASS(CLASS_NO, CLASS_TITLE, NO_OF_STUDENTS).

我需要写一个触发器来增加NO_OF_STUDENTS一个，如果一个新的学生加入到ENROLLMENT表，以CLASS_NO 。

Answer 1

你可以使用mysql触发器来做到这一点。 尝试类似的东西

CREATE TRIGGER 'database_name'.'after_insert_enrollment' AFTER INSERT ON 'ENROLLMENT' 
FOR EACH ROW
BEGIN
UPDATE class SET NO_OF_STUDENTS = NO_OF_STUDENTS +1 WHERE CLASS_NO = NEW.CLASS_NO;
END

Answer 2

如果删除注册，我也会减少no_of_studens 。

delimiter //
CREATE TRIGGER AFTER_INS_ENROLLMENT AFTER INSERT ON ENROLLMENT
FOR EACH ROW
BEGIN
    UPDATE CLASS SET NO_OF_STUDENTS = NO_OF_STUDENTS + 1 WHERE CLASS_NO = NEW.CLASS_NO;
END;//
delimiter;

delimiter //
CREATE TRIGGER AFTER_INS_ENROLLMENT AFTER DELETE ON ENROLLMENT
FOR EACH ROW
BEGIN
    UPDATE CLASS SET NO_OF_STUDENTS = NO_OF_STUDENTS - 1 WHERE CLASS_NO = NEW.CLASS_NO;
END;//
delimiter ;

Answer 3

你为什么要将它存储在唱片中？ 也许您应该使用Student Class_Id = @Class_Id中的Select COUNT（Student_Id）

Answer 4

你尝试过类似的东西：

delimiter |

CREATE TRIGGER testref BEFORE INSERT ON ENROLLMENT
  FOR EACH ROW BEGIN
    UPDATE CLASS SET NO_OF_STUDENTS = NO_OF_STUDENTS + 1 WHERE CLASS_NO = NEW.CLASS_NO;
  END;
|

delimiter ;