What are primary and foreign keys?

A primary key uniquely identifies each row in a table. It must be unique and cannot be NULL, and a table has only one. It can be a natural value or a surrogate key such as an auto-increment integer or a UUID. The database builds an index on it automatically in most systems.

A foreign key is a column (or set of columns) that references the primary key of another table. It enforces referential integrity: you cannot insert an order for a customer that does not exist, and you cannot delete a customer that still has orders, unless a rule says what to do. Actions such as ON DELETE CASCADE, SET NULL or RESTRICT define the behavior.

CREATE TABLE customers (
  id BIGINT PRIMARY KEY,
  email TEXT NOT NULL UNIQUE
);

CREATE TABLE orders (
  id BIGINT PRIMARY KEY,
  customer_id BIGINT NOT NULL REFERENCES customers (id)
);

Foreign keys are what make joins between tables meaningful.