1.数据类型 2.数据表的引擎
数据表
mysql> show tables;
+------------------+
| Tables_in_market |
+------------------+
| customers_info |
| orders |
+------------------+
2 rows in set (0.00 sec)
遇到错误信息
mysql> alter table orders add constraint fk_orders foreign key(c_id) references customers_info(c_num);
ERROR 1215 (HY000): Cannot add foreign key constraint
首先想到可能类型不同，于是查看表结构
mysql> desc customers_info;
+----------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+------------------+------+-----+---------+-------+
| c_num | int(10) unsigned | NO | PRI | 0 | |
| c_name | varchar(70) | YES | | NULL | |
| c_birth | datetime | NO | | NULL | |
| c_phone | varchar(50) | YES | | NULL | |
| c_gender | char(1) | YES | | NULL | |
+----------+------------------+------+-----+---------+-------+
5 rows in set (0.02 sec) mysql> desc orders;
+--------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+------------------+------+-----+---------+----------------+
| o_num | int(11) | NO | PRI | NULL | auto_increment |
| o_date | date | YES | | NULL | |
| c_id | int(10) unsigned | YES | | NULL | |
+--------+------------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)
发现c_id和c_num类型是相同的，那就有可能是引擎出问题了，查看创建数据表时的引擎
mysql> show create table customers_info\G;
*************************** 1. row ***************************
Table: customers_info
Create Table: CREATE TABLE `customers_info` (
`c_num` int(10) unsigned NOT NULL DEFAULT '',
`c_name` varchar(70) DEFAULT NULL,
`c_birth` datetime NOT NULL,
`c_phone` varchar(50) DEFAULT NULL,
`c_gender` char(1) DEFAULT NULL,
PRIMARY KEY (`c_num`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8
1 row in set (0.00 sec) ERROR:
No query specified mysql> show create table orders\G;
*************************** 1. row ***************************
Table: orders
Create Table: CREATE TABLE `orders` (
`o_num` int(11) NOT NULL AUTO_INCREMENT,
`o_date` date DEFAULT NULL,
`c_id` int(10) unsigned DEFAULT NULL,
PRIMARY KEY (`o_num`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec) ERROR:
No query specified
发现customers_info表的引擎是MyISAM, 修改此表的引擎为InnoDB
mysql> alter table customers_info engine = innoDB;
Query OK, 0 rows affected (0.57 sec)
Records: 0 Duplicates: 0 Warnings: 0 mysql> alter table orders add constraint fk_orders foreign key(c_id) references customers_info(c_num);
Query OK, 0 rows affected (0.62 sec)
Records: 0 Duplicates: 0 Warnings: 0
返回Query OK表明外键建立成功了，查看一下
mysql> show create table orders\G;
*************************** 1. row ***************************
Table: orders
Create Table: CREATE TABLE `orders` (
`o_num` int(11) NOT NULL AUTO_INCREMENT,
`o_date` date DEFAULT NULL,
`c_id` int(10) unsigned DEFAULT NULL,
PRIMARY KEY (`o_num`),
KEY `fk_orders` (`c_id`),
CONSTRAINT `fk_orders` FOREIGN KEY (`c_id`) REFERENCES `customers_info` (`c_num`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)