How can fill the missing dates in google BigQuery - firebase

Question

I want to write a chart that shows the active users in firebase
I wrote this code
SELECT event_date, COUNT(DISTINCT user_pseudo_id) AS user_count
FROM `mark-3314e.analytics_197261162.events_*`
WHERE _TABLE_SUFFIX BETWEEN FORMAT_DATE('%Y%m%d', DATE_SUB(CURRENT_DATE(), INTERVAL 7 DAY)) AND FORMAT_DATE('%Y%m%d', CURRENT_DATE())
AND event_name = 'session_start'
GROUP BY event_date
ORDER BY event_date ASC
And this is the response
Row event_date user_count
1 20190617 1
2 20190621 3
is there any way to fill the missing dates between 21 and 17 with the previous data? like:
event_date user_count
20190617 1
20190618 1
20190619 1
20190620 1
20190621 3

Answer 1

You may join with a calendar table which contains the full date range of interest:
WITH dates AS (
SELECT '20190617' AS dt UNION ALL
SELECT '20190618' UNION ALL
SELECT '20190619' UNION ALL
SELECT '20190620' UNION ALL
SELECT '20190621'
)
SELECT
t1.dt AS event_date,
COUNT(DISTINCT t2.user_pseudo_id) AS user_count
FROM dates t1
LEFT JOIN `mark-3314e.analytics_197261162.events_*` t2
ON t1.dt = t2.event_date AND
t2._TABLE_SUFFIX BETWEEN FORMAT_DATE('%Y%m%d', DATE_SUB(CURRENT_DATE(), INTERVAL 7 DAY)) AND FORMAT_DATE('%Y%m%d', CURRENT_DATE())
AND t2.event_name = 'session_start'
GROUP BY
t1.dt
ORDER BY
t1.dt;
For a more general way to generate a date range in BigQuery, see this SO question.

Answer 2

Here is a possible solution using GENERATE_DATE_ARRAY function in BigQuery.
with data as (
SELECT parse_date('%Y%m%d', event_date) AS event_date, COUNT(DISTINCT user_pseudo_id) AS user_count
FROM `mark-3314e.analytics_197261162.events_*`
WHERE _TABLE_SUFFIX BETWEEN FORMAT_DATE('%Y%m%d', DATE_SUB(CURRENT_DATE(), INTERVAL 7 DAY)) AND FORMAT_DATE('%Y%m%d', CURRENT_DATE())
AND event_name = 'session_start'
GROUP BY event_date
ORDER BY event_date ASC
)
select dt as event_date, user_count from (
select user_count,
if(
previousdate is null,
generate_date_array(date, date_sub(nextdate, interval 1 day), interval 1 day),
generate_date_array(date, if(nextdate is null, date, date_sub(nextdate, interval 1 day)), interval 1 day)
) as dates
from (
select
lag(event_date) over(order by event_date) as previousdate,
event_date as date,
lead(event_date) over(order by event_date) as nextdate,
user_count
from data
)
), unnest(dates) dt