How to Retrieve Uncancelled Reservations from a Database with Reservation Logs?

Identifying Active Reservations in a Database

This article demonstrates two SQL query methods for retrieving uncancelled reservations from a database containing reservation and reservation_log tables. A reservation is considered cancelled only if a corresponding entry with change_type = 'cancel' exists in the reservation_log table.

Method 1: NOT IN Subquery

This approach uses a subquery to identify cancelled reservations and then excludes them from the main query's results:

<code class="language-sql">SELECT *
FROM reservation
WHERE id NOT IN (
  SELECT reservation_id
  FROM reservation_log
  WHERE change_type = 'cancel'
);</code>

The inner query selects all reservation_id values associated with cancellations. The outer query then returns all reservations whose id is not present in this cancellation list.

Method 2: LEFT JOIN with IS NULL Condition

This method employs a LEFT JOIN to combine the reservation and reservation_log tables. The JOIN condition ensures that only cancellations are considered. The WHERE clause filters for rows where no matching cancellation is found:

<code class="language-sql">SELECT r.*
FROM reservation r
LEFT JOIN reservation_log l ON r.id = l.reservation_id AND l.change_type = 'cancel'
WHERE l.id IS NULL;</code>

The LEFT JOIN returns all rows from the reservation table (r). If a matching cancellation exists, the corresponding reservation_log data (l) is included; otherwise, l.id will be NULL. The WHERE l.id IS NULL condition filters out rows with matching cancellations, effectively isolating the uncancelled reservations. Only the reservation data (r.*) is selected in the final result.

Both methods achieve the same outcome, providing a list of uncancelled reservations. The choice between them often depends on performance considerations specific to the database system and data volume.

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