sp_spaceused using SQL query

select
-- from first result set of 'exec sp_spacedused'
DB_NAME() as [database_name],
--DBSize = CAST((sf.dbsize + sf.logsize) / 128.0 AS numeric(20, 2)),
--ltrim(str((convert (dec (15,2),sf.dbsize) + convert (dec (15,2),sf.logsize)) * 8192 / 1048576,15,2) + ' MB') as [database_size],
UnallocatedSpace = CAST((CASE WHEN sf.dbsize >= pages.reservedpages THEN (sf.dbsize - pages.reservedpages) / 128.0 ELSE 0 END) AS numeric(20, 2)),
--ltrim(str((case when sf.dbsize >= pages.reservedpages then
 --   (convert (dec (15,2),sf.dbsize) - convert (dec (15,2),pages.reservedpages))
 --   * 8192 / 1048576 else 0 end),15,2) + ' MB') as [unallocated space],

-- from second result set of 'exec sp_spacedused'
ReservedKB = pages.reservedpages * 8,
--ltrim(str(pages.reservedpages * 8192 / 1024.,15,0) + ' KB') as [reserved],
DataKB = pages.pages * 8,
--ltrim(str(pages.pages * 8192 / 1024.,15,0) + ' KB') as data,
IndexSizeKB = (pages.usedpages - pages.pages) * 8,
--ltrim(str((pages.usedpages - pages.pages) * 8192 / 1024.,15,0) + ' KB') as index_size,
UnusedKB = (pages.reservedpages - pages.usedpages) * 8,
--ltrim(str((pages.reservedpages - pages.usedpages) * 8192 / 1024.,15,0) + ' KB') as unused,

-- additional:
ltrim(str((convert (dec (15,2),sf.dbsize)) * 8192 / 1048576,15,2) + ' MB')  as dbsize,
ltrim(str((convert (dec (15,2),sf.logsize)) * 8192 / 1048576,15,2) + ' MB')  as logsize
FROM
(
  select
    sum(convert(bigint,case when status & 64 = 0 then size else 0 end)) as dbsize,
    sum(convert(bigint,case when status & 64 <> 0 then size else 0 end)) as logsize
  from dbo.sysfiles (nolock)
) sf,
(
  select
    sum(a.total_pages) as reservedpages,
    sum(a.used_pages) as usedpages,
    sum(
        CASE
            -- XML-Index and FT-Index internal tables are not considered "data", but is part of "index_size"
            When it.internal_type IN (202,204,211,212,213,214,215,216) Then 0
            When a.type <> 1 Then a.used_pages
            When p.index_id < 2 Then a.data_pages
            Else 0
        END
) as pages
  from sys.partitions p (nolock)
  join sys.allocation_units a (nolock) on p.partition_id = a.container_id
  left join sys.internal_tables it (nolock) on p.object_id = it.object_id
) pages
  --*/ 
--exec sp_spaceused