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當從父表中刪除該行時，如果子表中使用了該行的數(shù)據(jù)，那么MySQL將因FOREIGN KEY約束失敗而拋出錯誤。可以用“customer”和“orders”兩個表的例子來理解。這里，“customer”是父表，“orders”是子表。我們無法從“customer”表中刪除子表“orders”中使用的行。可以通過從父表中刪除值來演示，如下所示 –

mysql> Select * from Customer;
+----+--------+
| id | name   |
+----+--------+
| 1  | Gaurav |
| 2  | Raman  |
| 3  | Harshit|
| 4  | Aarav  |
+----+--------+
4 rows in set (0.00 sec)

mysql> Select * from orders;
+----------+----------+------+
| order_id | product  | id   |
+----------+----------+------+
| 100      | Notebook | 1    |
| 110      | Pen      | 1    |
| 120      | Book     | 2    |
| 130      | Charts   | 2    |
+----------+----------+------+
4 rows in set (0.00 sec)

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現(xiàn)在，假設我們嘗試從父表“customer”中刪除 id = 1 或 id = 2 的行（因為子表中使用了這兩個行），那么 MySQL 會拋出如下錯誤，因為外鍵約束失敗。

mysql> Delete from customer where id = 1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`query`.`orders`, CONSTRAINT `orders_ibfk_1` FOREIGN KEY (`id`)REFERENCES `customer` (`id`))

mysql> Delete from customer where id = 2;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`query`.`orders`, CONSTRAINT `orders_ibfk_1` FOREIGN KEY (`id`)REFERENCES `customer` (`id`))

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