How to Add a Foreign Key in SQL

A Foreign Key links a column (or columns) in one table to the Primary Key of another table to enforce referential integrity.

1. Add Foreign Key When Creating a Table

CREATE TABLE orders (    order_id INT PRIMARY KEY,    customer_id INT,    order_date DATE,    CONSTRAINT fk_customer FOREIGN KEY (customer_id) REFERENCES customers(customer_id));

2. Add Foreign Key to an Existing Table

Syntax:

ALTER TABLE child_tableADD CONSTRAINT constraint_name FOREIGN KEY (column_name)REFERENCES parent_table(parent_column);

Example:

ALTER TABLE ordersADD CONSTRAINT fk_customer FOREIGN KEY (customer_id)REFERENCES customers(customer_id);

Notes:

• The foreign key column(s) must have the same data type as the referenced primary key column(s).

• The parent table must exist before you create the foreign key.

• You can add ON DELETE or ON UPDATE actions:

ALTER TABLE ordersADD CONSTRAINT fk_customer FOREIGN KEY (customer_id)REFERENCES customers(customer_id)ON DELETE CASCADEON UPDATE CASCADE;

If you want the syntax for a specific SQL dialect or more examples, just let me know!