[RESOLVED] [sql express 2005] cannot obtain record in opposite way

[nUflAvOrS]

SELECT     dbo.MWorkOrder.MWORefNo, dbo.MWorkOrder.PWORefNo, dbo.MWorkOrder.DateOfService, dbo.MWorkOrder.Remarks, dbo.WOList.Category, 
                      dbo.WOList.Dept, dbo.WOList.Mac_Code, dbo.WOList.Status, dbo.BreakDownReport.BRRefNo
FROM         dbo.WOList INNER JOIN
                      dbo.MWorkOrder ON dbo.WOList.Ref_No = dbo.MWorkOrder.PWORefNo LEFT OUTER JOIN
                      dbo.BreakDownReport ON dbo.BreakDownReport.MWORefNo = dbo.MWorkOrder.MWORefNo
WHERE Status = 'Completed' AND Category LIKE '%' AND dbo.BreakDownReport.BRRefNo <> 'Null' ORDER BY dbo.MWorkOrder.MWORefNo

RETURN ME CORRECT RECORD WHICH dbo.BreakDownReport.BRRefNo <> 'Null'But why the SQL BELOW cannot return me the record where dbo.BreakDownReport.BRRefNo = 'Null'

SELECT     dbo.MWorkOrder.MWORefNo, dbo.MWorkOrder.PWORefNo, dbo.MWorkOrder.DateOfService, dbo.MWorkOrder.Remarks, dbo.WOList.Category, 
                      dbo.WOList.Dept, dbo.WOList.Mac_Code, dbo.WOList.Status, dbo.BreakDownReport.BRRefNo
FROM         dbo.WOList INNER JOIN
                      dbo.MWorkOrder ON dbo.WOList.Ref_No = dbo.MWorkOrder.PWORefNo LEFT OUTER JOIN
                      dbo.BreakDownReport ON dbo.BreakDownReport.MWORefNo = dbo.MWorkOrder.MWORefNo
WHERE Status = 'Completed' AND Category LIKE '%' AND dbo.BreakDownReport.BRRefNo = 'Null' ORDER BY dbo.MWorkOrder.MWORefNo

THANKS FOR HELPS !!