using aggregate tablix example
=Sum(IIF(Fields!IsDisplay.Value = "Yes", Fields!AvailPhysical.Value, Nothing))
sum all data on a tablix example
=Sum(IIf(Fields!BankAName.Value = "Standard Chartered Bank"
, Fields!Amount.Value
, Nothing)
, "DataSet1")
source :
http://stackoverflow.com/questions/18530541/ssrs-sum-expression-with-condition
