Question

What would be the best way to return the quantity of groups? Nothing special about my schema; table2 includes a FK referencing table1. The following seems to work. Any problems with it? Any better ways? Thanks

SELECT COUNT(*) FROM (
 SELECT COUNT(*)
 FROM table1 AS t1
 INNER JOIN table2 AS t2 ON t2.t1_id=t1.id
 WHERE t1.x=1 AND t2.y=1
 GROUP BY t2.z
) AS grouping

Answer 1

No, your solution is fine.

Another way would be

SELECT COUNT(DISTINCT t2.z) 
FROM table1 AS t1
INNER JOIN table2 AS t2 ON t2.t1_id=t1.id
WHERE t1.x=1 AND t2.y=1

This will also include NULLs.

From the manual:

Aggregate (summary) functions such as COUNT(), MIN(), and SUM() ignore NULL values. The exception to this is COUNT(*), which counts rows and not individual column values.