Write a query to retrieve unique records based on first name using the DISTINCT function.

You can use the DISTINCT keyword to eliminate duplicate first names:

SELECT DISTINCT first_name
FROM employees;

If you also need other columns but want uniqueness on first_name, you can use GROUP BY:

SELECT first_name, MIN(employee_id) AS sample_id
FROM employees
GROUP BY first_name;

The first query returns each unique first name once. The second query returns one row per first name, along with the smallest employee_id as a representative value. Both approaches run in O(n) time with respect to the number of rows, assuming appropriate indexes on first_name.