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Having

Function: HAVING

Purpose

The HAVING condition in Wonderful Relations Queries is used to filter results after the data has been processed, similar to the SQL HAVING clause. This differs from WHERE, which filters rows before aggregation and grouping.

A HAVING condition is useful when filtering needs to be applied after joins, grouping, or aggregations, allowing queries to first compile data before eliminating unwanted results.

Parameters

Condition

Defines the criteria for filtering the grouped results.

Sort Order

Determines the order in which HAVING conditions are applied.

How It Works

Unlike WHERE, which filters individual rows before grouping, HAVING operates after data aggregation. This makes it especially useful when working with grouped results or complex joins.

Example Use Case:

  • Suppose you want to filter results where the most recent event was more than 180 days ago, for example to clean up old data.
  • The HAVING clause ensures that only fully processed and grouped data is filtered.

SELECT
  wp_your_project_table_a.id,
  wp_your_project_table_a.active
FROM
  wp_your_project_table_a#!FILTERED_BY_LINK!#
JOIN wp_wr_link link_1_2_3_l
  ON (link_1_2_3_l.entity1 = child_entity
      AND link_1_2_3_l.id1 = wp_your_project_table_a.id
      AND link_1_2_3_l.entity2 = parent_entity)
JOIN (
 SELECT
    wp_your_project_table_b.id,
    wp_your_project_table_b.datum
  FROM
    wp_your_project_table_b
) wp_your_project_table_b
  ON (link_1_2_3_l.entity1 = child_entity
      AND link_1_2_3_l.id1 = wp_your_project_table_a.id
      AND link_1_2_3_l.id2 = wp_your_project_table_b.id )
 
WHERE wp_your_project_table_a.active = 1
GROUP BY wp_your_project_table_a.id
HAVING max(wp_your_project_table_b.datum) <= DATE_SUB(curdate(), INTERVAL 180 DAY);

Explanation

  1. Data Selection & Joining:

    • The query retrieves projects (wp_your_project_table_a) and their associated events (wp_your_project_table_b).
    • The JOIN ensures that each project is linked to its related event data.

  2. Initial Filtering with WHERE:

    • Filters for active projects only (wp_your_project_table_a.active = 1).

  3. Grouping:

    • The query groups results by wp_your_project_table_a.id.

  4. Applying HAVING for Final Filtering:

    • Filters out projects where the most recent event is older than 180 days (max(wp_your_project_table_b.datum) <= DATE_SUB(curdate(), INTERVAL 180 DAY)).

When to Use HAVING Instead of WHERE

ScenarioUse WHERE?Use HAVING?
Filtering before aggregation✅ Yes❌ No
Filtering grouped results❌ No✅ Yes
Filtering after a JOIN✅ Yes (if on raw data)✅ Yes (if filtering aggregates)
Filtering after a GROUP BY operation❌ No✅ Yes

Conclusion

The HAVING condition in Wonderful Relations Queries is an essential tool for post-aggregation filtering, allowing precise control over grouped results. By using HAVING alongside WHERE, GROUP BY, and JOIN, complex filtering logic can be efficiently implemented.

This method ensures that filtering occurs at the correct stage of query execution, preventing premature exclusions and improving data integrity. 🚀

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