外鍵約束
自帶的引擎只有innoDB引擎支持外鍵，外鍵要求父表必須有對應列的索引，子表會自動創建索引
下面是兩個表country國家，字表city
創建國家表
last_update語句的意思是時間戳不為空，默認插入（第三條插入表明以實際插入數值為准）和更新時間為當前時間
primary key 有兩個作用，一是約束作用（constraint），用來規范一個存儲主鍵和唯一性，但同時也在此key上建立了一個主鍵索引
CREATE TABLE country(
country_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
country VARCHAR(20),
last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ONUPDATE CURRENT_TIMESTAMP,
PRIMARY KEY(country_id)
);
INSERT INTO country VALUES(1,'china',NOW());
insert into country values(2,'japan',curdate());
insert into country values(3,'usa','2007-11-20 11:11:50');
mysql> select * from country;
+------------+---------+---------------------+
| country_id | country | last_update |
+------------+---------+---------------------+
| 1 | china | 2019-07-12 09:30:16 |
| 2 | japan | 2019-07-12 00:00:00 |
| 3 | usa | 2007-11-20 11:11:50 |
+------------+---------+---------------------+
3 rows in set (0.00 sec
創建城市表
KEY idx_fk_coutry_id (country_id) country_id為索引，可以不要，因為創建外鍵會創建該列的索引
添加外鍵的語句：創建約束fk_city_country，外鍵 FOREIGN KEY 所在列是本表country_id，級聯REFERENCES的是表country的country_id列，country表不能單獨刪除，級聯更新
CREATE TABLE city(
city_id INT UNSIGNED NOT NULL auto_increment,
city VARCHAR(20) NOT NULL,
last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
country_id INT UNSIGNED NOT NULL,
PRIMARY KEY (city_id),
KEY idx_fk_coutry_id (country_id),
CONSTRAINT fk_city_country FOREIGN KEY (country_id) REFERENCES country (country_id) ON DELETE RESTRICT ON UPDATE CASCADE
)ENGINE=INNODB default CHARSET=UTF8;
INSERT INTO city values(1,'nanjing',NOW(),1);
mysql> select * from city;
+---------+---------+---------------------+------------+
| city_id | city | last_update | country_id |
+---------+---------+---------------------+------------+
| 1 | nanjing | 2019-07-12 09:32:28 | 1 |
+---------+---------+---------------------+------------+
1 row in set (0.00 sec)
更新外鍵和刪除操作
#更新country
mysql> update country set country_id=2 where country_id=1;
Query OK, 1 row affected (0.05 sec)
#更新后查詢,city級聯更新
mysql> select * from country;
+------------+---------+---------------------+
| country_id | country | last_update |
+------------+---------+---------------------+
| 2 | japan | 2019-07-12 00:00:00 |
| 3 | usa | 2007-11-20 11:11:50 |
| 10 | china | 2019-07-12 09:34:32 |
+------------+---------+---------------------+
3 rows in set (0.00 sec)
mysql> select * from city;
+---------+---------+---------------------+------------+
| city_id | city | last_update | country_id |
+---------+---------+---------------------+------------+
| 1 | nanjing | 2019-07-12 09:32:28 | 10 |
+---------+---------+---------------------+------------+
1 row in set (0.00 sec)
#刪除city
mysql> delete from country where country_id=1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`test`.`city`, CONSTRAINT `fk_city_country` FOREIGN KEY (`country_id`) REFERENCES `country` (`country_id`) ON DELETE RESTRICT ON UPDATE CA SCADE)