What does sql foreign key constraint mean?

Foreign key constraints specify that there must be a reference relationship between tables to ensure data integrity, consistency, and reference integrity. Specific functions include: data integrity: foreign key values ​​must exist in the main table to prevent the insertion or update of illegal data. Data consistency: When the main table data changes, foreign key constraints automatically update or delete related data to keep them synchronized. Data reference: Establish relationships between tables, maintain reference integrity, and facilitate tracking and obtaining related data.

Meaning of SQL Foreign Key Constraints

A foreign key constraint is a relationship between database tables that specifies that one column or set of columns (foreign keys) must refer to the column (primary key) of another table (primary table). The purpose is to ensure the consistency and integrity of the data.

effect

Foreign key constraints have several main functions:

• Data Integrity: Foreign key constraints maintain data integrity by ensuring that foreign key values ​​exist in the main table to prevent data from being inserted or updated.
• Data consistency: When data in the main table changes, foreign key constraints automatically cascadingly update or delete relevant foreign key data to keep the data synchronized.
• Data Reference: Foreign key constraints help establish relationships between tables and maintain reference integrity, allowing you to easily track and get relevant data.

type

There are two main types of SQL foreign key constraints:

• A pure foreign key: The foreign key refers to a single column of the main table.
• Refer to foreign keys: Foreign keys refer to multiple columns of the main table.

grammar

In a SQL CREATE TABLE statement, you can use the FOREIGN KEY clause to define foreign key constraints. The syntax is as follows:

 <code class="sql">FOREIGN KEY (column_name) REFERENCES table_name (primary_key_column_name)</code>

in:

• column_name is a foreign key column.
• table_name is the main table name.
• primary_key_column_name is the primary key column in the main table.

Example

For example, if we have a table called Orders and a main table called Products , we can create the following foreign key constraint:

 <code class="sql">CREATE TABLE Orders ( order_id INT PRIMARY KEY, product_id INT NOT NULL, FOREIGN KEY (product_id) REFERENCES Products (product_id) );</code>

In addition, the key constraint ensures that each product_id value in Orders table exists in the Products table.

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