What is a foreign key?
A foreign key is a column that points to the primary key of another table, linking related rows together.
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A foreign key is a column whose values must match a primary key in another table. If an orders table has a customer_id column, each value must be a real customer. It is a reference like a footnote number: the footnote only makes sense if the numbered note actually exists at the bottom of the page.
Foreign keys are what make a relational database relational. They let you JOIN orders with customers, and they protect data: the database refuses to insert an order for customer 999 if no such customer exists, and can delete or block related rows when the parent goes away (ON DELETE CASCADE). Diagrams of tables and their foreign keys are called ER diagrams.
CREATE TABLE orders (
id SERIAL PRIMARY KEY,
customer_id INT REFERENCES customers (id)
);See also: What is a primary key? and What is SQL?
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See also
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