SQL - MOSTRAR MAX,MIN de varias columnas

select nombre,Company,isnull([4],0) as 'ABRIL',isnull([2],0) as 'FEBRERO',isnull([9],0) as 'SETIEMBRE',

'TOTAL' = (isnull([4],0)+isnull([2],0)+isnull([9],0)),PROMEDIO = ((isnull([4],0)+isnull([2],0)+isnull([9],0)))/3,

'MAXIMO' = (CASE WHEN isnull([4],0) > isnull([2],0) AND isnull([4],0) > isnull([9],0)  THEN isnull([4],0) ELSE

 CASE WHEN isnull([2],0) > isnull([4],0) AND isnull([2],0) > isnull([4],0)  THEN isnull([2],0) ELSE

 CASE WHEN isnull([9],0) > isnull([4],0) AND isnull([9],0) > isnull([2],0)  THEN isnull([9],0) ELSE 0

 END END END)

 from

(select e.firstname 'nombre',c.CompanyName 'Company',od.UnitPrice 'CANTIDAD'

,DATEPART(MONTH,O.OrderDate) as DATA_MES

from Employees e,Customers c,orders o,[Order Details] od

where

o.CustomerID = c.CustomerID and

o.EmployeeID = e.EmployeeID and

od.OrderID = o.OrderID

group by e.firstname,c.CompanyName,od.UnitPrice,O.OrderDate

) AS MESTABLE

PIVOT(

SUM(CANTIDAD)

FOR DATA_MES IN ([4], [2], [9])

)AS PIV_MES