Foreign Key in SQL

A foreign key is a database constraint that links a child table to a parent table by referencing the parent’s primary key, ensuring only valid, related data can be inserted.

What Is a Foreign Key in SQL?

Foreign keys are constraints that create a parent-child relationship between tables. The child table stores the foreign-key column, and each value must match an existing primary-key value in the parent table.

Why Do Databases Need Foreign Keys?

Databases need foreign keys to enforce referential integrity, preventing orphan records and guaranteeing that relationships remain consistent as data changes.

How Do I Create a Foreign Key?

Create a foreign key with the REFERENCES clause in a CREATE TABLE or ALTER TABLE statement. Always index the foreign-key column for fast joins.ALTER TABLE orders
ADD CONSTRAINT fk_orders_customer
FOREIGN KEY (customer_id)
REFERENCES customers(id);


How Does a Foreign Key Enforce Integrity?

When you insert, update, or delete data, the database checks the foreign-key constraint. If the referenced parent key does not exist, the operation fails, protecting data correctness.

What Are Cascade Actions?

Cascade actions like ON DELETE CASCADE and ON UPDATE CASCADE automatically propagate parent changes to child rows, reducing manual cleanup code.

When Should I Avoid a Foreign Key?

Avoid foreign keys in staging tables or high-volume log tables where speed outweighs integrity, but always document the trade-offs and add validation elsewhere.

Best Practices for Foreign Keys

Use clear constraint names, index both sides, prefer surrogate primary keys, and add cascade rules thoughtfully to prevent accidental mass deletes.

How Can I Inspect Foreign Keys in Galaxy?

Galaxy’s table sidebar shows foreign-key relationships instantly. Click a column to see its parent table, preview data, and generate JOIN templates with the AI copilot.