Problem

Your job is to find the most efficient way to query a database for a list of all customers who live in a given state and have made a purchase within the past 30 days.
Input:
State (e.g. "IL")
Output:
List of customers who live in the given state and have made a purchase within the past 30 days.

Solution

/*

SELECT * FROM customers
WHERE state = 'IL'
AND purchase_date > DATE_SUB(NOW(), INTERVAL 30 DAY)

This is the optimal solution because it uses the index on the state column to find all customers in the given state, and then filters out the ones who have not made a purchase in the past 30 days.

*/