Win-back orders after a long gap

The growth team wants to find win-back orders: orders placed more than 7 days after the same customer's previous order.

From food_orders, return customer_id, order_id, order_date, prev_order_date and days_since_prev for every such order. A customer's first order has no previous order and must not appear. Order by customer_id, then order_date.

1. Step 1: `LAG(order_date)` looks one row back inside each customer (`PARTITION BY customer_id`) after sorting by date.
2. Step 2: Window functions cannot be used in `WHERE`, so compute the previous date in a CTE and filter in the outer query.
3. Step 3: Subtracting two `DATE` values in PostgreSQL returns a whole number of days. For a first order `prev_order_date` is NULL, so the comparison is unknown and the row is dropped automatically.

WITH gaps AS (
    SELECT customer_id,
           order_id,
           order_date,
           LAG(order_date) OVER (
               PARTITION BY customer_id
               ORDER BY order_date, order_id
           ) AS prev_order_date
    FROM food_orders
)
SELECT customer_id,
       order_id,
       order_date,
       prev_order_date,
       order_date - prev_order_date AS days_since_prev
FROM gaps
WHERE order_date - prev_order_date > 7
ORDER BY customer_id, order_date;