SELECT SSN,
       LName,
       FName,
       MInit,
       COALESCE(SUM(Hours), 0) AS Known_Hrs,
       CASE
           WHEN SUM(Hours) IS NULL THEN 'Yes'
           ELSE 'No'
       END AS Unknown_Hrs,
       CASE
           WHEN SUM(Hours) > 40 THEN 'Yes'
           WHEN (COALESCE(SUM(Hours), 0) + COALESCE(SUM(CASE WHEN Hours IS NULL THEN 1 END) * 40, 0)) <= 40 THEN 'No'
           ELSE '?'
       END AS Overtime
FROM Employee
JOIN Works_On ON SSN=ESSN
GROUP BY SSN;