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What Are Composite Keys

What Is a Composite Key?

Combination of Columns: A composite key is a primary key in a database table that consists of two or more columns. Together the values in these columns uniquely identify each row (record) in the table.

Think of It as a Detailed Address: While a simple primary key could be like a house number, a composite key is more like the combination of house number, street, city and zip code giving you a highly specific identification.

Why Use Composite Keys?

Unique Identification in Complex Relationships:

Many real world scenarios need more than one column to guarantee uniqueness.

For example, in an order tracking table neither order ID nor product ID alone would be unique but the combination creates a specific identifier for each order item.

Natural Representation of Data

Composite keys reflect the natural structure of the data itself, making the database design more intuitive.

Enforcing Key Database Rules

Uniqueness: A composite key makes sure each row is unique.

Non null: Composite keys like any primary key disallow null values (missing data) in their component columns.

Example: Order Details Table

 

Composite Keys Table

Composite Key: The combination of OrderID and ProductCode is important here to find each row (order item).

Key Points (pun intended)

Component Order: The order of columns within the composite key can be significant depending on how the data is queried or indexed.

Database Design: Composite keys are prevalent and an important concept in relational database design.

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