mysql整数约束_MySQL外键约束 – 整数列
您可能希望将外键设置为接受NULL值,并使用NULL而不是0值.
从概念上讲,NULL表示缺少未知值.如果你的行“没有/需要值”,我相信NULL非常适合.
是的,NULL值不会破坏您的外键约束.
让我们建立一个基本的例子:
CREATE TABLE parents (
id int PRIMARY KEY,
value int
) ENGINE = INNODB;
CREATE TABLE children (
id int PRIMARY KEY,
parent_id int,
FOREIGN KEY (parent_id) REFERENCES parent (id)
) ENGINE = INNODB;
然后:
INSERT INTO parents VALUES (1, 100);
Query OK, 1 row affected (0.00 sec)
INSERT INTO children VALUES (1, 1);
Query OK, 1 row affected (0.00 sec)
INSERT INTO children VALUES (2, 0);
ERROR 1452 (23000): A foreign key constraint fails
INSERT INTO children VALUES (2, NULL);
Query OK, 1 row affected (0.00 sec)
SELECT * FROM children;
+——+—————-+
| id | parent_id |
+——+—————-+
| 1 | 1 |
| 2 | NULL |
+——+—————-+
2 rows in set (0.01 sec)
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