触发器-MySQL

© 版权声明：本文为博主原创文章，转载请注明出处

1.需求

　　- 统计同一所学校的学生人数

2.建表

　　- 学生表（student）

DROP TABLE IF EXISTS student;
CREATE TABLE `student` (
	`id` INT NOT NULL AUTO_INCREMENT,
	`name` VARCHAR(100),
	`age` SMALLINT,
	`school` VARCHAR(200) NOT NULL,
	PRIMARY KEY (`id`)
)ENGINE = INNODB DEFAULT CHARSET = UTF8

　　- 学校人数表（school_num）

DROP TABLE IF EXISTS school_num;
CREATE TABLE `school_num` (
	`school` VARCHAR(200) NOT NULL,
	`num` INT DEFAULT 0,
	PRIMARY KEY (`school`)
)ENGINE = INNODB DEFAULT CHARSET = UTF8;

3.触发器

　　- add_school_num

-- 新增时更新学校人数
DROP TRIGGER IF EXISTS add_school_num;
CREATE TRIGGER add_school_num
AFTER INSERT ON `student` FOR EACH ROW
BEGIN
	IF (NEW.school IN (SELECT school FROM school_num)) THEN
		UPDATE school_num SET num = num + 1 WHERE school = NEW.school;
	ELSE
		INSERT INTO school_num (school, num) VALUES (NEW.school, 1);
	END IF;
END;

　　- delete_school_num

-- 删除时更新学校人数
DROP TRIGGER IF EXISTS delete_school_num;
CREATE TRIGGER delete_school_num
AFTER DELETE ON `student` FOR EACH ROW
BEGIN
	IF (OLD.school IN (SELECT school FROM school_num)) THEN
		UPDATE school_num SET num = num -1 WHERE school = OLD.school;
	END IF;
END;

　　- update_school_num

-- 更新是更新学校人数
DROP TRIGGER IF EXISTS update_school_num;
CREATE TRIGGER update_school_num
AFTER UPDATE ON `student` FOR EACH ROW
BEGIN
	IF (NEW.school IN (SELECT school FROM school_num)) THEN
		UPDATE school_num SET num = num + 1 WHERE school = NEW.school;
	ELSE
		INSERT INTO school_num (school, num) VALUES (NEW.school, 1);
	END IF;
	IF (OLD.school IN (SELECT school FROM school_num)) THEN
		UPDATE school_num SET num = num - 1 WHERE school = OLD.school;
	END IF;
END;

4.测试

　　4.1 插入数据

　　　　4.1.1 执行SQL

INSERT INTO `student` (name, age, school) VALUES ('张三', 22, '北京大学');
INSERT INTO `student` (name, age, school) VALUES ('李四', 25, '北京大学');
INSERT INTO `student` (name, age, school) VALUES ('王五', 21, '清华大学');
INSERT INTO `student` (name, age, school) VALUES ('赵六', 19, '北邮大学');

　　　　4.1.2 student表数据

　　　　4.1.3 school_num表数据

　　4.2 删除数据

　　　　4.2.1 执行SQL

DELETE FROM `student` WHERE name = '赵六';

　　　　4.2.2 student表数据

　　　　4.2.3 school_num表数据

　　4.3 更新数据（更新为已存在的学校）

　　　　4.3.1 执行SQL

UPDATE `student` SET school = '北邮大学' WHERE name = '王五';

　　　　4.3.2 student表数据

　　　　4.3.3 school_num表数据

　　4.4 更新数据（更新为不存在的学校）

　　　　4.4.1 执行SQL

UPDATE `student` SET school = '传媒大学' WHERE name = '李四'

　　　　4.4.2 student表数据

　　　　4.4.3 school_num表数据