What does mysql foreign key constraint mean?

MySQL foreign key constraints refer to a constraint that limits the relationship between tables. You can define a column in one table, and this column will reference a column in another table. This Association relationships ensure data integrity and consistency.

The operating system of this tutorial: Windows 10 system, mysql version 8.0, Dell G3 computer.

MySQL foreign key constraints refer to a constraint that limits the relationship between tables. It can define a column in one table that will reference a column in another table. This correlation ensures data integrity and consistency.

MySQL is a widely used relational database management system, generally used for data storage and management of network applications. MySQL foreign key constraints refer to a mechanism used to ensure data integrity and manage data relationships. In MySQL, due to the relationship between tables, the use of foreign key constraints can limit operations such as data deletion and update, thereby ensuring Consistency and correctness of data throughout the system.

The foreign key itself is a special column that creates a connection between two tables through the foreign key column to form an association relationship. In MySQL, certain conditions need to be met to create foreign key constraints. For example, both tables must use the engine type InnoDB, and the foreign key column must be of the same type as the primary key column of the associated table. When creating a foreign key constraint, you can define an index on the primary key column of the associated table to improve query speed.

Using foreign key constraints can bring many benefits, such as avoiding data redundancy and errors, preventing data inconsistency, and improving data integrity and correctness. In MySQL, foreign key constraints can also implement cascade delete and update operations, that is, when a record in the main table is deleted or updated, the related records in the slave table can be automatically deleted or updated, thereby simplifying system maintenance and management.

Using foreign key constraints can achieve the following points:

1. Prevent deletion of records in the parent table

CREATE TABLE 表名(
id INT PRIMARY KEY,
...
);
CREATE TABLE 子表名称(
id INT PRIMARY KEY,
rid INT,
...
CONSTRAINT fk_table1 FOREIGN KEY(rid) REFERENCES 表名(id)
);

In this code, use the FOREIGN KEY and REFERENCES keywords Make an association. Using foreign key constraints in the child table to reference the parent table will prevent the rows in the child table from being deleted when a record in the parent table is deleted.

2. Prevent the insertion of invalid values

CREATE TABLE 表名(
id INT PRIMARY KEY,
...
);
CREATE TABLE 子表名称(
id INT PRIMARY KEY,
rid INT,
...
CONSTRAINT fk_table1 FOREIGN KEY(rid) REFERENCES 表名(id)
);

In this code, use the FOREIGN KEY and REFERENCES keywords to associate. When a foreign key constraint is used in a child table to reference a parent table, attempts to insert a value into the child table that does not exist in the parent table will be rejected.

3. Ensure data integrity and consistency

CREATE TABLE 表名(
id INT PRIMARY KEY,
...
);
CREATE TABLE 子表名称(
id INT PRIMARY KEY,
rid INT,
...
CONSTRAINT fk_table1 FOREIGN KEY(rid) REFERENCES 表名(id)
);

In this code, the FOREIGN KEY and REFERENCES keywords are used for association. Using foreign key constraints in the child table to reference the parent table ensures that the rid column in the child table must be obtained from the id column of the parent table. This correlation can ensure data integrity and consistency.

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