SELECT date_trunc('day',id.datetime) AS Дата,
MIN(channel35.val) AS ГЛ_V,
MIN(channel36.val) AS ГП_V
FROM
((SELECT datetime FROM cnldata) id LEFT JOIN
(SELECT datetime, val FROM cnldata WHERE cnlnum=35) channel35 ON channel35.datetime=id.datetime LEFT JOIN
(SELECT datetime, val FROM cnldata WHERE cnlnum=36) channel36 ON channel36.datetime=id.datetime)
WHERE
id.datetime BETWEEN '2017-01-01 09:00:00' AND '2018-01-03 09:10:00'
GROUP BY date_trunc('day',id.datetime)
ORDER BY date_trunc('day',id.datetime) ASC