How to group by time bucket in ClickHouse and fill missing data with nulls/0s

Question

Suppose I have a given time range. For explanation, let\'s consider something simple, like whole year 2018. I want to query data from ClickHouse as a sum aggregation for eac

Answer 1

You can generate zero values using the "number" function. Then join your query and zero values using UNION ALL and already according to the obtained data we make a GROUP BY.

So, your query will look like:

SELECT SUM(metric),
       time
  FROM (
        SELECT toStartOfQuarter(toDate(1514761200+number*30*24*3600))  time,
               toUInt16(0) AS metric
          FROM numbers(30)

     UNION ALL 

          SELECT toStartOfQuarter(created_at) AS time,
               metric
          FROM mytable
         WHERE created_at >= toDate(1514761200)
           AND created_at >= toDateTime(1514761200)
           AND created_at <= toDate(1546210800)
           AND created_at <= toDateTime(1546210800)
       )
 GROUP BY time
 ORDER BY time

note toUInt16(0) - zero values must be of the same type as metrics