? Alternate Key in SQL

An Alternate Key is any candidate key not chosen as the Primary Key in a table.

? Understanding with Definitions

• Candidate Key: A column (or combination of columns) that can uniquely identify each row.

• Primary Key: The candidate key chosen to uniquely identify rows.

• Alternate Key: The candidate key(s) not selected as the primary key.

? Example Table: students

Candidate Keys:

• student_id

• email

• roll_no

If we choose:

• Primary Key = student_id

• Then Alternate Keys = email, roll_no

? Declaring an Alternate Key

In SQL, you enforce an alternate key using a UNIQUE constraint.

CREATE TABLE students (  student_id INT PRIMARY KEY,  email VARCHAR(100) UNIQUE,  roll_no INT UNIQUE,  name VARCHAR(50));

Here:

• student_id is the Primary Key

• email and roll_no are Alternate Keys (with UNIQUE constraint)

? Why Use Alternate Keys?

• To ensure uniqueness on other columns besides the primary key.

• Helps prevent duplicate data (e.g., no duplicate emails or roll numbers).

• Can be used to define foreign key relationships in other tables.

Let me know if you want to practice with queries or explore composite alternate keys too!