MySQL - 触发器

触发器

创建表，并添加数据

创建学科表，并添加数据

CREATE TABLE course(
    id INT(3) PRIMARY KEY auto_increment,
    name VARCHAR(32),
    count INT(3) DEFAULT 0
)
INSERT INTO course(name) VALUES('Java');
INSERT INTO course(name) VALUES('Python');
INSERT INTO course(name) VALUES('HTML');

创建班级表，并添加数据

CREATE TABLE class(
    id INT(3) PRIMARY KEY auto_increment,
    name VARCHAR(32),
    count INT(3) DEFAULT 0
)
INSERT INTO class(name) VALUES('2401javaee');
INSERT INTO class(name) VALUES('2402javaee');
INSERT INTO class(name) VALUES('2403javaee');
INSERT INTO class(name) VALUES('2401python');
INSERT INTO class(name) VALUES('2402python');
INSERT INTO class(name) VALUES('2401html');
INSERT INTO class(name) VALUES('2402html');

创建学生表，并添加数据

CREATE TABLE student(
    id INT(3) PRIMARY KEY auto_increment,
    name VARCHAR(32),
    course_id INT(3),
    class_id INT(3)
)
INSERT INTO student(name,course_id,class_id) VALUES('马智威',1,3);
INSERT INTO student(name,course_id,class_id) VALUES('江星谊',1,3);
INSERT INTO student(name,course_id,class_id) VALUES('aaa',1,1);
INSERT INTO student(name,course_id,class_id) VALUES('bbb',2,4);
INSERT INTO student(name,course_id,class_id) VALUES('ccc',2,5);
INSERT INTO student(name,course_id,class_id) VALUES('ddd',2,5);

需求：添加学生，并更新学科表和班级表的数据

创建后置触发器

监听学生表的插入操作，一旦学生表插入数据之后，就更新学科表和班级表的数据

delimiter xx -- 设置结束符为xx

TRIGGER -- 触发器

AFTER -- 后置触发

new.字段 -- 新增的字段数据

delimiter xx
CREATE TRIGGER tri01 AFTER INSERT ON student FOR EACH ROW
    BEGIN
        UPDATE course SET count=count+1 WHERE id=new.course_id;
        UPDATE class SET count=count+1 WHERE id=new.class_id;
    END xx
delimiter ;

需求：删除学生，并更新学科表和班级表的数据

创建前置触发器

监听学生表的删除操作，一旦学生表删除数据之前，先更新学科表和班级表的数据

BEFORE - 前置触发

old.字段 -- 原有的字段数据

delimiter xx
CREATE TRIGGER tri02 BEFORE DELETE ON student FOR EACH ROW
    BEGIN
        UPDATE course SET count=count-1 WHERE id=old.course_id;
        UPDATE class SET count=count-1 WHERE id=old.class_id;
    END xx
delimiter ;
​
DELETE FROM student WHERE id=6;

需求：更新学生信息，并更新学科表和班级表的数据

创建前置触发器

监听学生表的更新操作，在学生表更新数据之后，更新学科表和班级表的数据

delimiter xx
CREATE TRIGGER tri03 AFTER UPDATE ON student FOR EACH ROW
    BEGIN
        UPDATE course SET count=count-1 WHERE id=old.course_id;
        UPDATE course SET count=count+1 WHERE id=new.course_id;
        UPDATE class SET count=count-1 WHERE id=old.class_id;
        UPDATE class SET count=count+1 WHERE id=new.class_id;
    END xx
delimiter ;
​
UPDATE student SET course_id=3,class_id=7 WHERE name='马智威';

删除触发器

DROP TRIGGER tri01;
DROP TRIGGER tri02;
DROP TRIGGER tri03;
​