i'm not new to this board but had to create a new log in due to a new role with a different company.  Please excuse my newness LOL. 

I have the following piece of code and for the life of me can't figure out why count () will work but sum(fieldname) won't.  I haven't written code in some time so a refresher would be helpful.  I'm trying to sum all the child records sph value and place this value on the parent obejct in a field called sph.  what am I missing.

trigger Add_Sales_Actuals on Sales_Actuals__c(after insert, after update) {
 
    //Student_Call_Log are the related records
    //Student is the "main" record
    //Idea is to update a field on the main record whenever an edit is made to a related record   
    
    Sales_Actuals__c [] sa = Trigger.new;    
    String bapid = null;
    
    bapid = sa[0].BA_Position__c;        
    if(sa[0].Reporting_date__c != null  )  
        {               
            integer i = [select sum(sph__c) from Sales_Actuals__c where BA_Position__c = :bapid 
                                                                  ];           
            // update the master record  
          
           BA_Position__c [] baps =[select id, SPH__c     
                                          from BA_Position__c where id = :bapid];
                          
                            
               baps[0].SPH__c = i;               
             
             // write to database    
                 
             update baps[0];      
             i = 0;     
           }
 }