I want to use the aggregation function

I am trying use AggregationType.Sum for in source code schema for my web service but its not summing the the totals as i wanted to do. It only sends the first records Amount rather than finding the all the records with that condition match and total them up. Can anyone direct me to documentation of implementing this? 

 var esq = new EntitySchemaQuery(UserConnection.EntitySchemaManager, "UsrExample");
             
var totalSum = esq.AddColumn("UsrTotal");

totalSum .SummaryType = AggregationType.Sum; // Set aggregation to sum
             
// Added filters: 
           
FinalAmount= sessionCollection[0].GetTypedColumnValue(totalSum.Name);