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Ravi Vishwakarma

IT-Hardware & Networking

Ravi Vishwakarma is a dedicated Software Developer with a passion for crafting efficient and innovative solutions. With a keen eye for detail and years of experience, he excels in developing robust software systems that meet client needs. His expertise spans across multiple programming languages and technologies, making him a valuable asset in any software development project.

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Ravi Vishwakarma

Ravi Vishwakarma

25-Mar-2026

In SQL Server, keys are used to uniquely identify records in a table and to establish relationships between tables. They play a crucial role in maintaining data integrity, consistency, and efficient querying.

What is a Key?

A key is a column or combination of columns that uniquely identifies a row in a table or creates a relationship between tables.

Types of Keys in SQL Server

1. Primary Key

A Primary Key uniquely identifies each record in a table.

Features:

  • Cannot contain NULL values
  • Must be unique
  • Only one primary key per table

Example:

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    Name VARCHAR(100),
    Email VARCHAR(100)
);

2. Foreign Key

A Foreign Key is used to link two tables. It refers to the primary key in another table.

Features:

  • Maintains referential integrity
  • Can have duplicate values
  • Can contain NULL (if allowed)

Example:

CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    EmployeeID INT,
    FOREIGN KEY (EmployeeID) REFERENCES Employees(EmployeeID)
);

3. Unique Key

A Unique Key ensures that all values in a column are unique.

Features:

  • Allows one NULL value (in SQL Server)
  • Can have multiple unique keys in a table

Example:

ALTER TABLE Employees
ADD CONSTRAINT UQ_Email UNIQUE (Email);

4. Composite Key

A Composite Key is a combination of two or more columns used to uniquely identify a record.

Example:

CREATE TABLE StudentCourses (
    StudentID INT,
    CourseID INT,
    PRIMARY KEY (StudentID, CourseID)
);

5. Candidate Key

A Candidate Key is a column (or set of columns) that can qualify as a primary key.

Example:

  • Email
  • EmployeeID
  • One of these is selected as the primary key.

6. Alternate Key

An Alternate Key is a candidate key that is not chosen as the primary key.

Example:
If EmployeeID is primary key, then Email can be an alternate key.

7. Super Key

A Super Key is a set of one or more columns that uniquely identifies a record.

Example:

  • EmployeeID
  • (EmployeeID + Name)
  • Both are super keys, but only minimal ones become candidate keys.

Importance of Keys

  • Ensure data uniqueness
  • Maintain referential integrity
  • Improve query performance (with indexing)
  • Prevent duplicate or inconsistent data
  • Help define relationships between tables

Best Practices

  • Always define a Primary Key
  • Use Foreign Keys to maintain relationships
  • Keep keys simple and minimal
  • Avoid using large columns (like VARCHAR) as keys
  • Use Composite Keys only when necessary

Conclusion

Keys are the backbone of relational databases in SQL Server. They help maintain structure, ensure accuracy, and enable efficient data relationships. A proper understanding of keys is essential for designing scalable and reliable database systems.

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