Use column alias in GROUP BY

I have a query where I chain two columns. Why does the alias **not** work in the GROUP BY clause but in ORDER BY clause, it does work? How can I write the select correctly? SELECT KOS_VER_ID AS "Vertrag" , WHR_ISO_3_CODE AS "Waehrung" , KOS_KOA_ST_KZN || ' - ' || ST_LANGBEZ_EN as "Kostenart" , SUM (KOS_BETRAG) AS "Summe pro KOA" FROM KOSTEN , WAEHRUNG , SCHLUESSELTABELLE WHERE KOSTEN.KOS_VERHI_WHR_ID = WAEHRUNG.WHR_ISO_ID AND KOSTEN.KOS_KOA_ST_KZN = SCHLUESSELTABELLE.ST_ID AND KOS_VER_ID in (2509, 2510, 2511) GROUP BY KOS_VER_ID , WHR_ISO_3_CODE , KOS_KOA_ST_KZN || ' - ' || ST_LANGBEZ_EN ORDER BY "Vertrag" , "Kostenart" ;