SQL Composite Key: Understanding Multi-Column Uniqueness

SQL Composite Key

A composite key is a combination of two or more columns in a table that together uniquely identify each row. Unlike a single-column primary key, a composite key ensures uniqueness only when all columns in the key are considered together. Individually, these columns do not guarantee uniqueness.

In real-world scenarios, certain datasets require multiple attributes to form a unique identity. A primary key formed by combining more than one column is known as a composite key.

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Key Characteristics of Composite Keys

• A composite key consists of two or more columns.
• It ensures uniqueness only when combined.
• The columns can be of different data types.
• It can act as a primary key or candidate key.

SQL Syntax for Defining a Composite Key

CREATE TABLE TABLE_NAME  
(COLUMN_A DATA_TYPE_A,  
 COLUMN_B DATA_TYPE_B,  
 COLUMN_C DATA_TYPE_C,  
 PRIMARY KEY (COLUMN_A, COLUMN_B));

Example: Creating a Composite Key in Different Databases

MySQL

CREATE TABLE STUDENT_COURSE  
(StudentID INT,  
 CourseID VARCHAR(50),  
 EnrollmentDate DATE,  
 PRIMARY KEY (StudentID, CourseID));

Oracle

CREATE TABLE EMPLOYEE_PROJECT  
(EmpID INT,  
 ProjectID VARCHAR(30),  
 StartDate DATE,  
 PRIMARY KEY (EmpID, ProjectID));

SQL Server

CREATE TABLE ORDER_DETAILS  
(OrderID INT,  
 ProductID NVARCHAR(50),  
 Quantity INT,  
 PRIMARY KEY (OrderID, ProductID));

Why Use a Composite Key?

1. Eliminates Redundancy: Prevents duplicate records.
2. Ensures Data Integrity: Helps maintain uniqueness across multiple attributes.
3. Optimizes Queries: Enables efficient multi-column indexing and lookups.
4. Reflects Real-World Relationships: Many scenarios require multi-attribute uniqueness (e.g., a student enrolling in multiple courses).

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Conclusion

Understanding composite keys is essential for database design when single-column primary keys are insufficient. They provide a practical way to enforce data integrity in multi-attribute datasets.

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