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Practice

Primary Keys

A primary key is a column (or combination of columns) that uniquely identifies each row in a table. It ensures that each row has a unique key value and that the key cannot contain NULL.


Why Use a Primary Key?​

Primary keys are used to:

  • Guarantees each record is unique
  • Prevents duplicate rows
  • Enables accurate referencing between tables (e.g. linking clients to orders)

Defining a Primary Key​

You can define a primary key when creating a table by using the PRIMARY KEY constraint.

Create orders table with primary key
CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  client_id INT,
  amount REAL,
  order_date TEXT
);

In this example, order_id is the primary key, and each order must have a unique ID.


Composite Primary Key​

Sometimes, one column isn't enough to uniquely identify a row. You can combine columns to form a composite primary key.

Composite primary key
CREATE TABLE order_items (
  order_id INT,
  product_id INT,
  quantity INT,
  PRIMARY KEY (order_id, product_id)
);

This ensures that each product appears only once in an order, preventing duplicates for the same product in the same order.

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