Select Customers Who Have Placed Orders in the Last 30 Days

SELECT
    c.customer_id,
    c.first_name,
    c.last_name,
    c.email
FROM
    customers c
WHERE EXISTS (
    SELECT 1
    FROM orders o
    WHERE o.customer_id = c.customer_id
      AND o.order_date >= CURRENT_DATE - INTERVAL '30 days' -- PostgreSQL/SQL Server. For MySQL: DATE_SUB(CURRENT_DATE, INTERVAL 30 DAY)
);

How it works: This query uses a correlated subquery with the EXISTS clause to identify customers who have placed an order in the last 30 days. For each customer in the `customers` table, the subquery checks if there's at least one corresponding order in the `orders` table that meets the date criterion. This is an efficient way to filter records based on the existence of related data without aggregating all related rows.