mysql数据库的常用操作-主键

• 关于主键                                                                                

• 外键一个特殊的索引，用于关键两个表，只能是指定的内容

• mysql> create table class(
      -> id  int not null primary key,
      -> name char(16));
  Query OK, 0 rows affected (0.02 sec)
   
   
  CREATE TABLE `student2` (
    `id` int(11) NOT NULL,
    `name` char(16) NOT NULL,
    `class_id` int(11) NOT NULL,
    PRIMARY KEY (`id`),
    KEY `fk_class_key` (`class_id`),
    CONSTRAINT `fk_class_key` FOREIGN KEY (`class_id`) REFERENCES `class` (`id`)
  ) 

   1 此时如果class 表中不存在id 1,student表也插入不了，这就叫外键约束
   2 mysql> insert into student2(id,name,class_id) values(1,'alex', 1);
   3 ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`testdb`.`student2`, CONSTRAINT `fk_class_key` FOREIGN KEY (`class_id`) REFERENCES `class` (`id`))
   4  
   5  
   6  
   7 mysql> insert into class(id,name) values(1,"linux");
   8 Query OK, 1 row affected (0.01 sec)
   9  
  10 mysql> insert into student2(id,name,class_id) values(1,'alex', 1);
  11 Query OK, 1 row affected (0.00 sec)
  12  
  13  
  14 ＃如果有student表中跟这个class表有关联的数据，你是不能删除class表中与其关联的纪录的
  15 mysql> delete from class where id =1;
  16 ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`testdb`.`student2`, CONSTRAINT `fk_class_key` FOREIGN KEY (`class_id`) REFERENCES `class` (`id`))