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448. JOIN using Composite Key

Understand how composite keys can be used to connect records across tables.

📘 Lesson 448 of 728 ⏱ 12 min 🎯 Intermediate

A composite key consists of two or more columns that together uniquely identify a record or relationship.

Some database designs require multiple columns to uniquely identify a business record.

Use ProductID and StoreID together to identify inventory for a specific product at a specific store.

SQL 
SELECT
    i.ProductID,
    i.StoreID,
    i.Quantity,
    p.ProductName
FROM Inventory i
INNER JOIN Products p
    ON i.ProductID = p.ProductID;

Composite keys are common in inventory, banking, regional reporting, mapping tables, and transaction systems.

  • A composite key uses multiple columns.
  • Columns together can identify a unique relationship.
  • JOIN conditions may use all key columns.
  • Composite keys are common in bridge and transaction tables.
  • Correct key design prevents incorrect matches.
What is a composite key?

A composite key is a key made up of two or more columns that together identify a record.

Why are composite keys useful?

They are useful when no single column can uniquely identify a business record or relationship.

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