mysql> CREATE TABLE class ( `id` int NOT NULL AUTO_INCREMENT, `name` varchar(128) DEFAULT NULL, `teacher` varchar(64) DEFAULT NULL, `count` int DEFAULT 0, UNIQUE KEY `id` (`id`) ); #创建班级表 class
insert into class values(101, '萌新一班', 'Martin', 0),(102, '萌新二班', 'Rock', 0),(103, '萌新三班', 'Janny', 0);
#创建学生表
CREATE TABLE `student` ( `id` int NOT NULL AUTO_INCREMENT UNIQUE, `name` varchar(64) DEFAULT NULL, `class_id` int DEFAULT NULL, `sex` enum('F','M') DEFAULT NULL );
#创建触发器,学生增加则count增一
create trigger tri_insert_student after insert on student for each row update class set count=count+1 where class.id = NEW.class_id;
#向学生表增加多条数据
insert into student values(1,'小花',101,'M'),(2,'小红',102, 'F'),(3,'小军',102,'F'),(4,'小白',101,'F');
#创建触发器,删除学生count减一
create trigger tri_delete_student after delete on student for each row update class set count=count-1 where id = OLD.class_id;
#删除触发器
DROP TRIGGER trigger_name;