194. Daily active users report for new market

easy snap


We recently launched the Snap app in this new country on 2022-03-01, and now we want to run some data analysis.


  • Write a query to return DAU from 2022-03-01 to 2022-03-31
  • If there is no DAU on that date, return 0.
  • DAU: a user who has at least one event on that day.

Table 1: dates

Calendar dates from 01/01/2019 to 12/31/2025.

 col_name | col_type
 year     | smallint
 month    | smallint
 date     | date

Table 2: snap_session

   col_name     | col_type
 user_id        | bigint
 event          | text 
 dt             | date -- date of the event

Sample results

    date    | count
 2022-03-01 |     9
 2022-03-02 |     7
 2022-03-03 |     0
 2022-03-04 |     6
 2022-03-05 |    15

Expected results

Solution postgres

SELECT D.date, count(DISTINCT S.user_id) dau
FROM dates D
LEFT JOIN snap_session S
ON D.date = S.dt
WHERE D.date >= '2022-03-01'
AND D.date <= '2022-03-31'


This query is selecting data from two tables, "dates" and "snap_session". It is counting the number of unique user IDs that occurred on each day between March 1, 2022, and March 31, 2022.

The "LEFT JOIN" statement ensures that all dates from the "dates" table are included in the final result, even if there are no matching records in the "snap_session" table.

The "WHERE" clause is used to filter the results to only include dates within the specified time frame.

The "GROUP BY" clause groups the results by date.

Finally, the "ORDER BY" clause orders the results by date in ascending order.

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