How to Efficiently Retrieve Unmodified Reservations from a Database?

Database Query for Unmodified Reservations

This article addresses the efficient retrieval of unmodified reservations from a database with two tables: reservation and reservation_log. A simple WHERE clause isn't ideal for finding uncancelled reservations; a more sophisticated approach is needed.

Method 1: Exclusion using NOT IN

One method uses negation to exclude cancelled reservations. We identify cancelled reservations in reservation_log and exclude their IDs from the reservation table:

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

Method 2: Optimized Query with LEFT JOIN

For better performance, especially with indexed columns, a LEFT JOIN is recommended:

<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>

This LEFT JOIN returns all rows from the reservation table. Matching rows from reservation_log (where a cancellation exists) are included. The WHERE l.id IS NULL clause filters the results, returning only reservations without a matching cancellation entry in reservation_log.

Both methods effectively retrieve uncancelled reservations. The LEFT JOIN approach generally offers superior performance. Choosing the best method depends on database size and indexing.

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