Composite Key SQL

Galaxy Glossary

What is a composite key, and how is it used in a relational database?

A composite key is a database constraint that combines multiple columns to uniquely identify a row in a table. It's crucial for ensuring data integrity and avoiding duplicate entries.
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Description

In relational databases, a primary key uniquely identifies each row in a table. Sometimes, a single column isn't enough to guarantee uniqueness. A composite key, or compound key, is a primary key composed of multiple columns working together. This approach is essential when a single column isn't sufficient to uniquely identify a record. For example, in an order table, you might have customer ID and order date. A single customer could place multiple orders on different dates. Therefore, a composite key using both customer ID and order date is necessary to uniquely identify each order. This ensures that no two orders from the same customer on the same date are accidentally duplicated or overwritten. Composite keys are particularly useful in scenarios where a single column isn't sufficient to represent the unique identity of a record, such as in situations involving multiple attributes that need to be considered together. This approach is crucial for maintaining data integrity and preventing data redundancy.

Why Composite Key SQL is important

Composite keys are essential for maintaining data integrity in relational databases. They prevent duplicate entries and ensure that each record is uniquely identifiable, which is crucial for accurate data analysis and reporting. This is vital for applications that need to track and manage complex relationships between data.

Example Usage


CREATE TABLE Orders (
    CustomerID INT,
    OrderDate DATE,
    OrderID INT PRIMARY KEY,
    OrderTotal DECIMAL(10, 2)
);

CREATE TABLE Customers (
    CustomerID INT PRIMARY KEY,
    FirstName VARCHAR(50),
    LastName VARCHAR(50)
);

-- Inserting data into the Orders table using a composite key
INSERT INTO Orders (CustomerID, OrderDate, OrderID, OrderTotal) VALUES
(1, '2024-01-15', 101, 100.50),
(1, '2024-01-20', 102, 150.00),
(2, '2024-01-18', 103, 75.25);

-- Selecting data using the composite key
SELECT * FROM Orders WHERE CustomerID = 1 AND OrderDate = '2024-01-15';

Common Mistakes

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