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

As an alternative for numbers() function in some cases range and array functions can be useful.

Example: for each pair of (id1,id2) dates from the previous 7 days should be generated.

SELECT
  id1,
  id2,
  arrayJoin(
    arrayMap( x -> today() - 7 + x, range(7) )
  ) as date2
FROM table
WHERE date >= now() - 7
GROUP BY id1, id2

The result of that select can be used in UNION ALL to fill the 'holes' in data.

SELECT id1, id2, date, sum(column1)
FROM (
  SELECT
    id1,
    id2,
    date,
    column1 
  FROM table
  WHERE date >= now() - 7

  UNION ALL 

  SELECT
    id1,
    id2,
    arrayJoin(
      arrayMap( x -> today() - 7 + x, range(7) )
    ) as date2,
    0 as column1
  FROM table
  WHERE date >= now() - 7
  GROUP BY id1, id2
)
GROUP BY id1, id2, date
ORDER BY date, id1, id2