sql server - Pivot with datediff -

how remove zeros , replace them null values considering not 0 value?

my working data

id       class     startdate      enddate      1        high        1/1/15       2/1/15        1         low        5/1/15       6/1/15        1         mid        6/1/15      6/10/15   2     mid-low        6/1/15      6/10/15 

my query below

 select y.id, y.startdate, y.enddate, isnull(y.high,'') high, isnull(y.mid,'') mid, isnull(y.low,'') low, isnull(y.mid-low,'') mid-low,  ( select id, class, datediff(day, startdate, enddate) days @test ) x pivot ( max(days) class in (high, low, med) ) y 

this returns set value below

  id     high       mid    low    mid-low    1       30         9     30          0    2        0         0      0         30 

having post answer show code. unable reproduce op's results.

this code:

declare @table table (   cola char(1)  ,colb int  );   insert @table(cola,colb) values ('a',null);  select * ( select cola,colb @table )x pivot ( sum(colb) cola in (a,b)  --same result if sum() changed max() ) y; 

results in:

a     b null  null 

so op doing more example illustrates getting zeros in his/her results.