Crystal Reports Conditions on two separate tables

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CrystalReports

I am developing a Crystal Report with the condition in two sepatare table, using .NET. The condition is below check the present of value from another table and display results based on condition. When I apply to the crystal report, does not return any value.

I am using Crystal Report 2008

if {BAQReportResult.PartTran.TranType} = 'STK-CUS' then  
Sum({BAQReportResult.ShipDtl.OurInventoryShipQty})