Como as respostas acima mencionadas, você precisará de uma junção externa e algum tipo de tabela de calendário. Isso não foi testado, mas acho que funcionará para você:
with dateCTE as
(
select cast('2012-01-01' as datetime) dateValue -- start date
union all
select DateAdd(mm, 1, dateValue)
from dateCTE
where dateValue < '2012-12-30' -- end date
)
SELECT
MONTH(dateCTE.dateValue) AS 'MONTH',
COUNT(CASE WHEN @Worker_ID1 IS NULL OR @Worker_ID1 = Worker.ID THEN 1 END) AS 'Worker1',
COUNT(CASE WHEN @Worker_ID2 IS NULL OR @Worker_ID2 = Worker.ID THEN 1 END) AS 'Worker2',
COUNT(CASE WHEN @Worker_ID3 IS NULL OR @Worker_ID3 = Worker.ID THEN 1 END) AS 'Worker3',
COUNT(CASE WHEN @Worker_ID4 IS NULL OR @Worker_ID4 = Worker.ID THEN 1 END) AS 'Worker4',
COUNT(CASE WHEN @Worker_ID5 IS NULL OR @Worker_ID5 = Worker.ID THEN 1 END) AS 'Worker5'
FROM dateCTE
LEFT JOIN Ord on MONTH(dateCTE.datevalue) = MONTH(Ord.OrdDate)
JOIN Prod ON Ord.Prod_ID = Prod.ID
JOIN ProdType ON Prod.ProdType_ID = ProdType.ID
JOIN Groupord ON Ord.Groupord_ID = Groupord.ID
JOIN Worker ON Groupord.Worker_ID = Worker.ID
WHERE (@Year = YEAR(Ord.OrdDate) or ORD.prod_id is null)
AND (@DrugType_ID IS NULL OR @ProdType_ID = ProdType.ID)
GROUP BY MONTH(dateCTE.dateValue)
OPTION (MAXRECURSION 0)
