Cannot delete or update a parent row: a foreign key constraint fails - How to solve MySQL error: The parent row has a foreign key constraint

How to solve the MySQL error: The parent row has foreign key constraints and requires specific code examples

When using the MySQL database for development, we often encounter an error message : "Cannot delete or update a parent row: a foreign key constraint fails" (Cannot delete or update a parent row: a foreign key constraint fails).

This error message usually occurs when we try to delete or update a row that is referenced by records in other tables. In this case, MySQL will prevent the delete or update operation on the parent row based on the foreign key constraint. In order to solve this problem, we need to handle foreign key constraints accordingly.

The following will provide you with some methods to solve this problem, with specific code examples.

1. Delete related child row data
   Before performing a delete or update operation, we need to delete the child row data related to the parent row. This can be achieved by setting the ON DELETE CASCADE attribute of the foreign key. When a parent row is deleted, all child rows that reference the parent row are also automatically deleted.

For example, suppose we have two tables: orders and order_items. The order_id field of the order_items table is a foreign key reference to the order in the orders table. When we want to delete an order in the orders table, we must first delete the sub-row data related to the order in the order_items table.

The specific code examples are as follows:

CREATE TABLE orders(
    id INT PRIMARY KEY,
    order_name VARCHAR(50)
);

CREATE TABLE order_items(
    id INT PRIMARY KEY,
    order_id INT,
    item_name VARCHAR(50),
    FOREIGN KEY (order_id) REFERENCES orders(id) ON DELETE CASCADE
);

In the above code, we set the order_id field to pair ## when creating the order_items table #ordersForeign key constraints of the table, and ON DELETE CASCADE is specified. In this way, when we delete an order in the orders table, the records in the order_items table related to the order will also be automatically deleted.

Delete after releasing the foreign key constraint
2. If we do not want to set ON DELETE CASCADE, or cannot delete the child row data related to the parent row, we can first release the foreign key constraint, and then proceed delete operation.
   

The specific code examples are as follows:

ALTER TABLE order_items DROP FOREIGN KEY fk_order_items_order_id;

In the above code, we use the

ALTER TABLE statement to delete order_items pairs in the table ##ordersForeign key constraints of the table. Next, we can perform the delete operation as follows:

DELETE FROM orders WHERE id = 1;

When executing the above delete statement, MySQL will no longer verify the foreign key constraints related to the deleted row, thus Successfully deleted parent row.

Update the value of the parent row

In some cases, we may only want to update the value of the parent row instead of deleting it. In this case, we need to update the foreign key references of all child rows related to that parent row before updating the parent row's value.
3. 
   
The specific code examples are as follows:

UPDATE order_items SET order_id = 2 WHERE order_id = 1;
UPDATE orders SET id = 2 WHERE id = 1;

In the above code, we first update the foreign key reference in the

order_items

table to the previously referenced parent row ID The order_id field value of the record that is 1 is updated to 2. Then, we update the value of the record with ID 1 in the orders table to 2. In this way, we successfully updated the value of the parent row.

Summary:

When encountering MySQL error: "Cannot delete or update a parent row: a foreign key constraint fails", we can delete the child row data related to the parent row, Solve this problem by deleting or updating the value of the parent row after releasing the foreign key constraint. Choose the corresponding processing method according to the specific business needs and data operation conditions, and write reasonable code to handle the problem of foreign key constraints.

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