Efficiency of public conventional thermal power production, EU28

PREFIX owl: <http://www.w3.org/2002/07/owl#>
PREFIX sdmx-measure: <http://purl.org/linked-data/sdmx/2009/measure#> 
PREFIX sdmx-dimension: <http://purl.org/linked-data/sdmx/2009/dimension#> 
PREFIX property: <http://rdfdata.eionet.europa.eu/eurostat/property#> 
PREFIX indic_nrg: <http://dd.eionet.europa.eu/vocabulary/eurostat/indic_nrg/>
PREFIX unit: <http://dd.eionet.europa.eu/vocabulary/eurostat/unit/>
PREFIX geo: <http://dd.eionet.europa.eu/vocabulary/eurostat/geo/>
PREFIX product: <http://dd.eionet.europa.eu/vocabulary/eurostat/product/>
PREFIX sdmx-attribute: <http://purl.org/linked-data/sdmx/2009/attribute#> 

SELECT DISTINCT
    year(?date) as ?year
    'EU28' as ?geo
    #sum(?nrg_100a_6000_5200)
    #sum(?nrg_100a_0000)
    round(( ( sum(?nrg_100a_6000_5200) ) / ( sum(?nrg_100a_0000) ) )*1000)/10.0 as ?efficiency_rate
  WHERE {{
    GRAPH <http://rdfdata.eionet.europa.eu/eurostat/data/nrg_110a.rdf.gz> { 
    {    
    _:nrg_100a sdmx-dimension:timePeriod ?date .
    _:nrg_100a sdmx-dimension:refArea geo:EU28 .
    _:nrg_100a sdmx-attribute:unitMeasure unit:KTOE .
    
    {
      _:nrg_100a property:product ?product . FILTER (?product in (product:6000, product:5200 ))
      _:nrg_100a property:indic_nrg indic_nrg:B_101121 . 
      _:nrg_100a sdmx-measure:obsValue ?nrg_100a_6000_5200 .
    } UNION {
      _:nrg_100a property:product product:0000 .
      _:nrg_100a property:indic_nrg indic_nrg:B_101021 . 
     _:nrg_100a sdmx-measure:obsValue ?nrg_100a_0000 .
    }
 }}
}}
GROUP BY ?date
ORDER BY ?date