This example applies to SSRS 2008
SELECT a.datafield1 As Col1, a.datafield2 As Col2, b.datafield6 As Col3 LEFT OUTER JOIN databaseB.dbo.Table4 b on b.datafield6 = a.datafield1 FROM databaseA.dbo.Table5 a UNION SELECT a.datafield7 As Col1, a.datafield8 As Col2, b.datafield12 As Col3 LEFT…