SELECT month,name, prem from t2, t1
where prem in (select max(prem) from t1) and t2.id = t1.Id_emp

SELECT month,name,MAX(prem) over(partition by month ) as Max_Prem from t1, t2
where t2.id = t1.Id_emp

SELECT TOP (1) WITH TIES
[month],name, prem 
FROM t2 INNER join t1 ON t2.id_emp = t1.id
ORDER BY rank() OVER (PARTITION BY [MONTH] ORDER BY prem DESC)

WITH CTE AS
(
  SELECT T.ID_EMP,T.MONTH,T.PREM,
  RANK()OVER(PARTITION BY T.MONTH ORDER BY T.PREM DESC)AS XCOL
  FROM T1 AS T
)
SELECT C.ID,C.MONTH,C.PREM
FROM CTE AS C
WHERE C.XCOL=1