What does fk mean in sql

FK in SQL is a foreign key constraint that maintains data integrity and consistency between two tables. By establishing logical relationships, ensuring that the data in the subtable references to valid data in the main table and preventing the deletion of the data referenced in the main table.

What is FK in SQL?

FK is the abbreviation for foreign key constraints. Foreign key constraints are database objects used in relational databases to maintain data integrity and consistency.

The role of foreign key constraints

Foreign key constraints establish a logical relationship between two tables, ensuring that the data in one table is associated with the data in the other table. It does the following:

• Data Integrity: Ensure that the data in the subtable references to valid data in the main table.
• Reference Integrity: Prevents the deletion of data referenced in the primary table from the subtable.
• Cascading operations: When the data in the main table changes, the corresponding operations in the subtable (such as cascading update or cascading deletion) will be automatically triggered.

The syntax of foreign key constraints

In SQL, the syntax of foreign key constraints is as follows:

 <code class="sql">ALTER TABLE [子表名称] ADD FOREIGN KEY ([子表列名称]) REFERENCES [主表名称] ([主表列名称])</code>

Example of foreign key constraints

Suppose we have the following two tables:

• Customers table contains customer information, where customer_id is the primary key.
• Orders table contains order information, where customer_id is a foreign key, which refers to customer_id in the Customers table.

To create a foreign key constraint, we can use the following SQL statement:

 <code class="sql">ALTER TABLE Orders ADD FOREIGN KEY (customer_id) REFERENCES Customers (customer_id)</code>

This foreign key constraint ensures that all customer_id in Orders table correspond to valid customers in the Customers table.

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