What is a primary key? Can it be composite? Provide an example of a composite primary key.

A primary key is a unique identifier for a record in a database table. It guarantees that each row can be uniquely identified.

A primary key can be composite, meaning it consists of multiple columns that together form a unique identifier.

Example of a composite primary key:

CREATE TABLE Orders (
  order_id INT,
  product_id INT,
  quantity INT,
  PRIMARY KEY (order_id, product_id)
);

Here, the uniqueness of the record is determined by the combination of order_id and product_id.