24. Second order within seven days
Asked in screens shaped like: Shopify, DoorDash, Uber
Retention wants customers whose second completed order landed within 7 days of their first completed order. Return customer_id and the days between those two orders, smallest customer_id first.
Requirements
- Columns: customer_id, days_between
- Only completed orders
- days_between is the day gap between 1st and 2nd completed order
- Keep rows where days_between <= 7
- Order by customer_id
Expected output
customer_id, days_between
Row order is graded, so ORDER BY matters here.
Tables this problem reads. The full warehouse is available in thesandbox.
orders
~200 rows
Order headers. One row per order, so amount is the order total — not a line total.
- order_idINTEGER— 1000–1199
- customer_idTEXT— C01–C12
- countryTEXT— US, DE, IN, UK, BR
- amountREAL
- statusTEXT— 'completed' or 'cancelled'
- ordered_atTEXT— YYYY-MM-DD HH:MM:SS
Pick a warehouse dialect above the editor. The engine is still SQLite; common syntax is rewritten before it runs.
SQLite cheat sheet
- Truncate to day
DATE(ordered_at) - Current date
DATE('now') - Conditional
CASE WHEN … THEN … ELSE … END - String aggregate
GROUP_CONCAT(col, ", ") - JSON field
json_extract(payload, '$.source') - Date difference
JULIANDAY(a) - JULIANDAY(b) - Filter a window
CTE, then WHERE rn = 1 - Median
ROW_NUMBER + COUNT(*) OVER ()
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