apply_filters_ref_array( 'posts_groupby', string $groupby, WP_Query $query )

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• If you come with MySQL knowledge, the GROUP BY clause is pretty useless without the ability to modify the SELECT statement.
• There is no SELECT filter since the query is supposed to return only the post data. The GROUP BY clause is set only when there are Custom Field Parameters for querying by post meta or Taxonomy Parameters for querying by taxonomy.
• The default posts_groupby is set to {$wpdb->posts}.ID, which means that even if there are multiple results because of multiple meta and taxonomy, they are grouped together by the post id.