FOREIGN KEY 约束的ON UPDATE CASCADE 和 ON DELETE CASCADE
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创建表studentcreate table student(id int PRIMARY KEY,name varchar(32),sex int NOT NULL,)
创建表coursecreate table course(id int PRIMARY KEY,name varchar(32) NOT NULL,id_s int,CONSTRAINT fk_student FOREIGN KEY (id_s) REFERENCES student(id) ON UPDATE CASCADE ON DELETE CASCADE)注意:
course表中的'id_s'列指向student表中的'id'列
student表中的'id'列是student表的primary key
course表中的'id_s'列是course表的foreign key
由于增加外键约束时添加了ON UPDATE CASCADE 和ON DELETE CASCADE
当用update 更新student表中'id'列时, course表中对应值也会被更新
当用delete 删除student表中某个记录时, course表中对应记录也会被删除
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