I have a travel logs table with vehicle IDs and travel dates. I want to compare whether two different vehicles have traveled on the same day. How would you write a SQL query to find pairs of vehicles that share the same travel date?

Use a self‑join on the travel_logs table, matching rows with the same travel_date but different vehicle_id values. To avoid duplicate pairs (e.g., 101‑102 and 102‑101), enforce an ordering condition such as vehicle_id1 < vehicle_id2. The query returns each unique pair once.

Example:

SELECT
  a.travel_date,
  a.vehicle_id AS vehicle_1,
  b.vehicle_id AS vehicle_2
FROM travel_logs a
JOIN travel_logs b
  ON a.travel_date = b.travel_date
 AND a.vehicle_id < b.vehicle_id
ORDER BY a.travel_date, vehicle_1, vehicle_2;

This produces a list of dates and the vehicle pairs that were on the same date. If you need to count how many days each pair shared, wrap the above in a subquery and group by the pair:

SELECT vehicle_1, vehicle_2, COUNT(*) AS shared_days
FROM (
  SELECT a.travel_date, a.vehicle_id AS vehicle_1, b.vehicle_id AS vehicle_2
  FROM travel_logs a
  JOIN travel_logs b
    ON a.travel_date = b.travel_date
   AND a.vehicle_id < b.vehicle_id
) t
GROUP BY vehicle_1, vehicle_2
ORDER BY shared_days DESC;

The approach runs in O(n log n) time due to the join, but for typical log sizes it is efficient.