SQL Alternate Key

Introduction

In SQL, an alternate key is a column or a combination of columns that can uniquely identify a record within a table, similar to a primary key. However, unlike the primary key, an alternate key may not be selected as the main identifier of a table. It provides an alternative means to uniquely identify a record when the primary key is not suitable or has not been defined.

Importance of Alternate Key

• Data Integrity: Alternate keys help maintain data integrity by ensuring that each record in a table has a unique identifier. This can prevent duplicate or conflicting data from being entered into the database.
• Relationships: Alternate keys can be used to establish relationships between tables, just like primary keys. Foreign keys can reference alternate keys, enabling data normalization and efficient query operations.
• Flexibility: Alternate keys provide flexibility in choosing a unique identifier for a table. They can be used when the primary key is not ideal or needs to be changed without impacting existing relationships or application logic.
• Performance: By using alternate keys in query conditions or join operations, database performance can be improved. They can provide more selective access paths and facilitate index usage.

Examples

Example 1: Alternate Key Definition

CREATE TABLE employees (
    emp_id INT,
    emp_email VARCHAR(100) UNIQUE, -- Alternate key
    emp_name VARCHAR(100),
    emp_department VARCHAR(50),
    emp_salary DECIMAL(10,2),
    ...
);

Example 2: Referencing Alternate Key

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    ...
    FOREIGN KEY (customer_id) REFERENCES customers (customer_id), -- Primary key reference
    FOREIGN KEY (order_number) REFERENCES invoices (order_number) -- Alternate key reference
);

Example 3: Query Using Alternate Key

SELECT *
FROM employees
WHERE emp_email = 'john.doe@example.com';

Conclusion

Alternate keys in SQL provide an additional means to uniquely identify records in a table. They ensure data integrity, improve query performance, and enhance database design flexibility. By using alternate keys, you can establish relationships between tables and choose suitable identifiers for your tables based on your requirements.