SELECT  `Datum`  AS 'Zeit',
COALESCE(AVG(nullif(`Kammer1`, 0)),0) AS Kammer1,
COALESCE(AVG(nullif(`Kammer2`, 0)),0) AS Kammer2,
COALESCE(AVG(nullif(`Kammer3`, 0)),0) AS Kammer3,
COALESCE(AVG(nullif(`Kammer4`, 0)),0) AS Kammer4,
COALESCE(AVG(nullif(`Kammer5`, 0)),0) AS Kammer5,
COALESCE(AVG(nullif(`Kammer6`, 0)),0) AS Kammer6
FROM `tk` WHERE 1 Group by  DATE_FORMAT(`Datum`, "%d-%m-%Y %H"), HOUR(Datum)%3  
ORDER BY `Zeit`  ASC;