Report builder: count in two different columns
You should be able to achieve the desired result with this query:
SELECT DATE_FORMAT(date_of_treatment, '%M %Y') as Date, COUNT(*) as 'Number of aneurysms', ( SELECT COUNT(*) FROM <?php echo $TABLE_PREFIX ?>aortic_aneurysm_disease WHERE type_of_treatment='Open repair' AND DATE_FORMAT(date_of_treatment, '%M %Y') = Date ) as 'Number of open repairs' FROM <?php echo $TABLE_PREFIX ?>aortic_aneurysm_disease GROUP BY Date ORDER BY YEAR(date_of_treatment) DESC, MONTH(date_of_treatment) DESC
You were already very close, the only thing to know is that a subquery (like how you were selecting the "Open repair" records) doesn't "know" anything about the main query around it, so we need to add some context for it to select the correct records. In this case, we've added some matching on the formatted Date column.
Let me know if this fixes your issue, or if you have any other questions!