mysql foreign key and delete

MySQL foreign key and deletion

MySQL is a commonly used relational database management system that is widely used and very powerful. In MySQL, foreign keys are a very important concept and play an important role in database design and data maintenance. In this article, we will introduce foreign keys in MySQL and how to use foreign keys to delete data in the database.

MySQL Foreign Key

Foreign Key (Foreign Key) refers to setting a key in one database table that points to the primary key of another table. If a table has a foreign key, then the main table record corresponding to each qualified record in this table must exist, otherwise related operations cannot be performed. This operation ensures the integrity and consistency of data between tables.

We illustrate the concept of foreign keys through the following example:

Create table person:

CREATE TABLE person (
    id INT PRIMARY KEY,
    name VARCHAR(20)
);

Create table card:

CREATE TABLE card (
    id INT PRIMARY KEY,
    person_id INT,
    card_no VARCHAR(20),
    FOREIGN KEY(person_id) REFERENCES person(id)
);

In table card , person_id is a foreign key, which points to the id field in the table person. The purpose of this foreign key is to ensure that the person_id in each card record exists in the person table, thus ensuring consistency and integrity.

Use foreign keys to delete

When we delete in the database, if there is a relationship between the tables, we need to ensure that the delete operation will not destroy the integrity and integrity of the database. consistency. In MySQL, this can be achieved using foreign keys.

When performing a deletion operation, if the record to be deleted exists in a table associated with a foreign key, you need to first delete the record in the associated table, and then delete the record in this table. This process can be achieved using ON DELETE CASCADE and ON DELETE SET NULL.

ON DELETE CASCADE operates by automatically classifying and deleting related records in this table when records in the associated table are deleted. Example:

CREATE TABLE person (
    id INT PRIMARY KEY,
    name VARCHAR(20)
);

CREATE TABLE card (
    id INT PRIMARY KEY,
    person_id INT,
    card_no VARCHAR(20),
    FOREIGN KEY(person_id) REFERENCES person(id) ON DELETE CASCADE
);

INSERT INTO person(id, name) VALUES(1, 'Tom');

INSERT INTO card(id, person_id, card_no) VALUES(1, 1, '123456');

DELETE FROM person WHERE id = 1;

In the above example, we first inserted a record into the person table, and then inserted a record into the card table. This record points to the record with id 1 in the person table. Finally, we used the DELETE statement to delete the record with id 1 in the person table. This deletion operation will also automatically delete the related records in the card table.

ON DELETE SET NULL operation is when the records in the related table are deleted, the related records in this table are set to NULL. Example:

CREATE TABLE person (
    id INT PRIMARY KEY,
    name VARCHAR(20)
);

CREATE TABLE card (
    id INT PRIMARY KEY,
    person_id INT,
    card_no VARCHAR(20),
    FOREIGN KEY(person_id) REFERENCES person(id) ON DELETE SET NULL
);

INSERT INTO person(id, name) VALUES(1, 'Tom');

INSERT INTO card(id, person_id, card_no) VALUES(1, 1, '123456');

DELETE FROM person WHERE id = 1;

In the above example, we first inserted a record into the person table, and then inserted a record into the card table. This record points to the record with id 1 in the person table. Finally, we used the DELETE statement to delete the record with id 1 in the person table. This deletion operation will automatically set the person_id in the related record in the card table to NULL.

Summary

MySQL foreign key is a very important concept and plays an important role in database design and data maintenance. Using foreign keys can ensure the integrity and consistency of data between tables. When performing a delete operation, you can use ON DELETE CASCADE and ON DELETE SET NULL to handle the association between tables to ensure database consistency.

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