The empty date in the table will cause an error in the QUARTER() function.

SELECT year(date), quarter(date), count(person_id), 
  FROM service ;
  WHERE NOT EMPTY(date) ;
  GROUP BY 1, 2 

>Another day, another SQL question:
>
>I need to get quarterly student counts, where a student could appear in multiple quarters, they need to be counted for each quarter.
>
>My first question is simply why does this work...
>
>

>SELECT MONTH(date) FROM service
>

>
>but this doesn't...
>
>

>SELECT QUARTER(date) FROM service
>

>
>The latter returns an error: Function argument, date, type, or count is invalid
>
>Other than the above, I'm wondering generally how best to tackle this problem. I'm want something like:
>
>

>SELECT count(person_id), quarter(date);
>  FROM service;
>  GROUP BY quarter(date)
>