• Use the following expression in the textbox :

    =LookupSet(Fields!PRODUCT_BRAND_CODE.Value, Fields!PRODUCT_BRAND_CODE.Value, Fields!PRODUCT_NUMBER.Value, "Product").Length

    From Ref: http://social.msdn.microsoft.com/Forums/en/sqlreportingservices/thread/135ef7c9-a4d0-4f16-9828-5ca8c2ee00ab

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