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?
- Eliminates Redundancy: Prevents duplicate records.
- Ensures Data Integrity: Helps maintain uniqueness across multiple attributes.
- Optimizes Queries: Enables efficient multi-column indexing and lookups.
- 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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