Why Does My LEFT JOIN Return Incorrect Counts in PostgreSQL?

PostgreSQL LEFT JOIN: Addressing Incorrect Count Results

When using a LEFT JOIN in PostgreSQL to count entries per organization, you might encounter a situation where only organizations with counts greater than zero are returned. This contradicts the expected behavior of a LEFT JOIN, which should include all organizations, regardless of whether they have matching entries in the joined table.

Correcting the LEFT JOIN Query

The problem typically arises from incorrectly placing filtering conditions. The following revised query demonstrates the correct approach:

<code class="language-sql">SELECT
  o.name AS organisation_name,
  COUNT(e.id) AS total_used
FROM
  organisations o
LEFT JOIN
  exam_items e
  ON e.organisation_id = o.id
  AND e.item_template_id = #{sanitize(item_template_id)}
  AND e.used
GROUP BY
  o.name
ORDER BY
  o.name;</code>

The crucial modification involves moving the e.used condition from the WHERE clause (where it acts as a filter after the join) to the ON clause (where it acts as a join condition). This ensures that only matching rows satisfying all conditions are included in the join itself, preventing the unintended filtering of organizations with zero counts.

Optimized Approach: Pre-Aggregation

For improved performance, particularly when dealing with large datasets, consider this alternative query:

<code class="language-sql">SELECT
  o.id,
  o.name AS organisation_name,
  COALESCE(e.total_used, 0) AS total_used
FROM
  organisations o
LEFT JOIN
  (
    SELECT
      organisation_id AS id,
      COUNT(*) AS total_used
    FROM
      exam_items
    WHERE
      item_template_id = #{sanitize(item_template_id)}
      AND used
    GROUP BY
      1
  ) AS e
  USING (id)
ORDER BY
  o.name,
  o.id;</code>

This method first aggregates the counts within a subquery, then performs the join. This pre-aggregation can significantly enhance efficiency, especially when a substantial portion of exam_items rows are included in the count. The COALESCE function handles cases where total_used is NULL (for organizations without matching entries), replacing it with 0 for consistency.

The above is the detailed content of Why Does My LEFT JOIN Return Incorrect Counts in PostgreSQL?. For more information, please follow other related articles on the PHP Chinese website!