Sql sum() varchar tipindeki miktar toplama

SELECT DISTINCT TOP 100
      t.[kod_ref]
      ,t.[urunadi]
      ,t.[fiyat]
     ,(SELECT SUM(cast(miktar as integer)) [kod_ref] from [CAGLAYAN_WEB].[dbo].[siparis] 
       
      where kod_ref = t.kod_ref) as sa

  FROM [CAGLAYAN_WEB].[dbo].[siparis]  t order by sa desc