how to take sum of Column in AX 2012...?????

Hello All,

I Would like to know how to take sum of one Column in AX2012...???

I am making a method then will take one parameter and will return a sum of that particular paramter.

when i run this

select SalesBalanceMST from

        salesTable

      Where salesTable.SalesID==custInvoiceJour.SalesID

       join custInvoiceJour

       Where custInvoiceJour.SalesType=="Sales order"

       && custInvoiceJour.CustGroup==CustGroupId

       && salesTable.SalesStatus=="Invoiced";

it runs fine and gives no error, But when i run this

select sum(SalesBalanceMST) from

        salesTable

      Where salesTable.SalesID==custInvoiceJour.SalesID

       join custInvoiceJour

       Where custInvoiceJour.SalesType=="Sales order"

       && custInvoiceJour.CustGroup==CustGroupId

       && salesTable.SalesStatus=="Invoiced";

This gives syntax error...

Please give any help it would be really helpfull...

Regards

Fahad Mirza