Detailed explanation of the role and usage of foreign keys in MySQL database

Detailed explanation of the role and usage of foreign keys in MySQL database

In the MySQL database, foreign keys are an important tool used to establish associations between tables. It can Ensures data integrity and provides a way to maintain relationships between tables. This article will introduce in detail the role and usage of foreign keys in the MySQL database and provide specific code examples.

1. The role of foreign keys

1. Ensure data integrity: Foreign keys can ensure that the data in the child table must exist in the parent table, avoiding errors caused by data inconsistency mistake.
2. Establish relationships between tables: Foreign keys can define relationships between tables, making querying and operating data more convenient and efficient.
3. Constrain data operations: Foreign keys can restrict operations on table data to ensure data consistency and correctness.

2. Usage of foreign keys

1. Define foreign keys when creating a table: When creating a table, you can establish an association between tables by defining foreign keys. relation. The syntax is as follows:

CREATE TABLE table name (
    column name data type,
    ...
    FOREIGN KEY (foreign key column name) REFERENCES parent table name (parent table column name)
    [ON DELETE CASCADE/SET NULL/RESTRICT/NO ACTION]
    [ON UPDATE CASCADE/SET NULL/RESTRICT/NO ACTION]
);

Among them, FOREIGN KEY is used to define foreign keys, REFERENCES specifies the parent table and parent table columns, ON DELETE and ON UPDATE is used to specify the operation strategy for deletion and update.

2. Modify the table structure and add foreign keys: If you need to add a foreign key to an existing table, you can use the ALTER TABLE statement to modify the table structure. The syntax is as follows:

ALTER TABLE subtable name
ADD CONSTRAINT foreign key name
FOREIGN KEY (foreign key column name) REFERENCES parent table name (parent table column name);

3. Delete foreign key: If you need to delete the foreign key constraints of the table, you can use ALTER TABLE statement to delete foreign keys. The syntax is as follows:

ALTER TABLE subtable name
DROP FOREIGN KEY foreign key name;

4. Use foreign key constraints: During data operations, MySQL will check the integrity of the data based on the definition of the foreign key. For example, when inserting data into a child table, if the inserted foreign key value does not exist in the parent table, the insertion will fail.

3. Specific code examples

The following uses a specific example to demonstrate the use of foreign keys in the MySQL database:

Suppose we have two tables, one is The student table (students) and the other is the grades table (scores) have a one-to-many relationship between them. We require that the student ID in the student grade table must exist in the student table.

1. Create students table:

CREATE TABLE students (
    student_id INT PRIMARY KEY,
    student_name VARCHAR(50)
);

2. Create a score table and define foreign keys:

CREATE TABLE scores (
    score_id INT PRIMARY KEY,
    student_id INT,
    score INT,
    FOREIGN KEY (student_id) REFERENCES students(student_id)
);

In the above example, we defined the student_id column in the grades table as a foreign key, which is associated with the student_id column in the student table. In this way, when inserting grade data, the system will automatically check the validity of the student ID.

Summary:

The foreign key in the MySQL database is a very important data relationship tool. It can ensure data integrity, establish relationships between tables, and constrain data. operate. Proper use of foreign keys can improve database operation efficiency and data consistency, which is an indispensable part of database design. I hope this article will help you understand the role and usage of foreign keys in the MySQL database.

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