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mysql delete foreign key

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Release： 2023-05-18 14:10:37

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MySQL method of deleting foreign keys

In MySQL, foreign keys are an important concept, which ensures data integrity and consistency. When we need to delete foreign keys, we need to pay attention to some details. This article will introduce the method of deleting foreign keys in MySQL and discuss some problems you may encounter.

Step one: View existing foreign keys

In MySQL, we can view existing foreign keys through the following statement:

SHOW CREATE TABLE table_name;

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Among them, table_name is to be viewed The table name of the foreign key. After executing this statement, MySQL will return a result set containing the table structure. In the result set we can find the definition of foreign keys. For example, the foreign key name in the following example is fk_order_customer, which joins the customer_id field of the orders table and the ## of the customers table #id field.

CREATE TABLE `orders` (
  `id` int(11) NOT NULL,
  `customer_id` int(11) NOT NULL,
  `order_number` varchar(45) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_order_customer` (`customer_id`),
  CONSTRAINT `fk_order_customer` FOREIGN KEY (`customer_id`) REFERENCES `customers` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

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Step 2: Delete foreign keys

There are two ways to delete foreign keys.

Method 1: Use the ALTER TABLE statement

We can use the ALTER TABLE statement to delete foreign keys. The following is the syntax:

ALTER TABLE table_name DROP FOREIGN KEY foreign_key_name;

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Among them, table_name is the name of the table to delete the foreign key, and foreign_key_name is the name of the foreign key to be deleted. For example, the following statement will delete the

fk_order_customer foreign key in the above example:

ALTER TABLE orders DROP FOREIGN KEY fk_order_customer;

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Method 2: Use the DROP INDEX statement

We can also use the DROP INDEX statement to delete the foreign key key. The following is the syntax:

ALTER TABLE table_name DROP INDEX index_name;

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Among them, table_name is the name of the table to delete the foreign key, and index_name is the name of the foreign key to be deleted. For example, the following statement will delete the

fk_order_customer foreign key in the example above:

ALTER TABLE orders DROP INDEX fk_order_customer;

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Both methods can be used to delete the foreign key. However, depending on how the foreign key is defined (called a "constraint" in MySQL), we need to use a different approach.

If the foreign key is defined as "foreign key constraint" (also called "relationship constraint"), we must use the ALTER TABLE statement to delete the foreign key. In the above example, the foreign key is defined as a "foreign key constraint", so we use the ALTER TABLE statement to delete it.

If the foreign key is defined as "index constraint", we can use the DROP INDEX statement to delete the foreign key. In MySQL, index constraints are automatically generated by "foreign key indexes". Note that if we only delete the foreign key index but not the foreign key constraint, the foreign key still exists, but it is no longer binding. In this case, we may need to use the ALTER TABLE statement to drop the foreign key constraint itself.

Step 3: Check the deletion operation

Before performing the deletion operation, we should first check whether the deletion operation will affect existing data. For example, if you delete a foreign key constraint, you may violate data integrity, cause the delete to fail, or break data consistency.

Here are some tips to check for delete operations:

In the above example, the foreign key we want to delete The constraint connects the

customer_id field of the orders table and the id field of the customers table. This means that the customer_id field of the orders table references the id field of the customers table. If we delete the foreign key constraint, the customer_id field of the orders table may refer to the id field of the customers table that does not exist. This is not allowed.

Therefore, before deleting the foreign key constraint, we must ensure that there are no records in the

customers table that are referenced by the orders table. A simple way is to check by the following statement:

SELECT * FROM customers WHERE id NOT IN (SELECT customer_id FROM orders);

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If this statement returns an empty result set, it means that there is no one in the

customers table that is referenced by the orders table records, we can safely remove the foreign key constraint. Otherwise, we need to first delete rows in the orders table that reference non-existent records in the customers table, and then delete the foreign key constraints.

In the above example, the foreign key constraint defines cascade delete and cascade update operations. This means that if we delete or update a record in the

customers table, then all rows in the orders table that refer to it will also be deleted or updated. Therefore, before removing foreign key constraints, we must consider the impact of cascading operations.

Here are some suggestions:

orders rows in the table , and then delete the foreign key constraint.

Summary

MySQL deleting foreign keys is a common operation and contains some details. We can use the ALTER TABLE statement or the DROP INDEX statement to delete foreign keys, but we need to pay attention to how the foreign keys are defined. Before performing a deletion operation, we should check whether the deletion operation will affect existing data, especially in terms of dependencies and cascading operations.

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