SELECT
    sum(c),
    toInt32((h - null::Nullable(DateTime)) / 3600) + 1 AS a
FROM
(
    SELECT count() AS c, h
    FROM ( SELECT now() AS h )
    WHERE toInt32((h - null::Nullable(DateTime)) / 3600) + 1 = 1
    GROUP BY h
)
GROUP BY a settings min_count_to_compile_expression=0;

DB::Exception: Cannot convert column 
`plus(toInt32(divide(minus(h, CAST(NULL, 'Nullable(DateTime)')), 3600)), 1)` 
because it is non constant in source stream but must be constant in result. (ILLEGAL_COLUMN)