Indexing and Different Types of Keys in SQL

In the field of database management systems (DBMS), SQL is the standard language for interacting with relational databases. To achieve efficient data retrieval, manipulation, and organization, SQL relies on indexing and keys—two fundamental mechanisms that underpin database optimization, performance, and integrity.

This essay examines the concept of indexing in SQL, explains its importance, and explores the different types of keys that shape relational database design.

What is Indexing in SQL?

Indexing in SQL functions much like an index in a book—it accelerates the search process. Instead of scanning every row in a table (a full table scan), the database can use an index to quickly pinpoint rows that meet specific query criteria.

Example: Without vs. With an Index

• Without Index:

SELECT * FROM Employees WHERE LastName = 'Sharma';

• The database examines each row in the Employees table to find matches.
• With Index:

CREATE INDEX idx_lastname ON Employees(LastName);

SELECT * FROM Employees WHERE LastName = 'Sharma';

• The database leverages the idx_lastname index to locate results much faster.

Illustration:
Imagine searching for a book in a vast library. Without an index, you would check every book one by one. With an index (like a catalog), you can go straight to the relevant shelf.

Types of Indexes in SQL

• Clustered Index:
  • Organizes and stores table rows based on the index key.
  • Each table can have only one clustered index.
  • Example:

CREATE CLUSTERED INDEX idx_empid ON Employees(EmployeeID);

• Non-Clustered Index:
  • Creates a separate structure pointing back to the original table data.
  • A table can have multiple non-clustered indexes.
  • Example:

CREATE NONCLUSTERED INDEX idx_dept ON Employees(Department);

• Unique Index:
  • Guarantees all values in the indexed column are unique.
  • Often created automatically with a primary key.
  • Example:

CREATE UNIQUE INDEX idx_email ON Employees(Email);

• Composite Index:
  • An index covering two or more columns.
  • Useful for queries filtering by multiple columns.
  • Example:

CREATE INDEX idx_name_dept ON Employees(LastName, Department);

Different Types of Keys in SQL

Keys are constraints that preserve data uniqueness and integrity, and define relationships between tables.

1. Primary Key

• Uniquely identifies each row in a table; cannot be NULL.
• Example:

CREATE TABLE Employees (   
 EmployeeID INT PRIMARY KEY,   
 FirstName VARCHAR(50),   
 LastName VARCHAR(50)
);

2. Foreign Key

• Links two tables, enforcing referential integrity.
• Example:

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

3. Unique Key

• Ensures all values in a column are unique; can allow a single NULL.
• Example:

ALTER TABLE Employees ADD CONSTRAINT uq_email UNIQUE (Email);

4. Composite Key

• A primary key consisting of two or more columns.
• Example:

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

5. Candidate Key

• Columns (or sets of columns) that could uniquely identify records. Only one becomes the primary key; others remain as candidates.

6. Alternate Key

• Candidate keys not chosen as the primary key.

Why Indexing and Keys Matter

Indexing significantly improves query performance by reducing search times, while keys ensure data accuracy and maintain relationships. Together, these features keep databases efficient, reliable, and scalable.