How to run SUM() OVER PARTITION BY for COUNT DISTINCT

Question

I'm trying to get the number of distinct users for each event at a daily level while maintainig a running sum for every hour. I'm using Athena/Presto as the query engine.

I tried the following query:

SELECT
    eventname,
    date(from_unixtime(time_bucket)) AS date,
    (time_bucket % 86400)/3600 as hour,
    count,
    SUM(count) OVER (PARTITION BY eventname, date(from_unixtime(time_bucket)) ORDER BY eventname, time_bucket) AS running_sum_count
FROM (
    SELECT 
        eventname,
        CAST(eventtimestamp AS bigint) - CAST(eventtimestamp AS bigint) % 3600 AS time_bucket,
        COUNT(DISTINCT moengageuserid) as count
    FROM clickstream.moengage
    WHERE date = '2020-08-20'
    AND eventname IN ('e1', 'e2', 'e3', 'e4')
    GROUP BY 1,2
    ORDER BY 1,2
);

But on seeing the results I realized that taking SUM of COUNT DISTINCT is not correct as it's not additive.

So, I tried the below query

SELECT
    eventname,
    date(from_unixtime(time_bucket)) AS date,
    (time_bucket % 86400)/3600 as hour,
    SUM(COUNT(DISTINCT moengageuserid)) OVER (PARTITION BY eventname, date(from_unixtime(time_bucket)) ORDER BY eventname, time_bucket) AS running_sum
FROM (
    SELECT
        eventname,
        CAST(eventtimestamp AS bigint) - CAST(eventtimestamp AS bigint) % 3600 AS time_bucket,
        moengageuserid
    FROM clickstream.moengage
    WHERE date = '2020-08-20'
    AND eventname IN ('e1', 'e2', 'e3', 'e4')
);

But this query fails with the following error:

SYNTAX_ERROR: line 5:99: ORDER BY expression '"time_bucket"' must be an aggregate expression or appear in GROUP BY clause

Answer 1

To calculate running distinct count you can collect user IDs into set (distinct array) and get the size:

cardinality(set_agg(moengageuserid)) OVER (PARTITION BY eventname, date(from_unixtime(time_bucket)) ORDER BY eventname, time_bucket) AS running_sum

This is analytic function and will assign the same value to the whole partition (eventname, date), you can aggregate records in upper subquery using max(), etc.

Answer 2

Count the first time a user appears for the running distinct count:

SELECT eventname, date(from_unixtime(time_bucket)) AS date,
       (time_bucket % 86400)/3600 as hour,
       COUNT(DISTINCT moengageuserid) as hour_cont,
       SUM(CASE WHEN seqnunm = 1 THEN 1 ELSE 0 END) OVER (PARTITION BY eventname, date(from_unixtime(time_bucket)) ORDER BY time_bucket) AS running_distinct_count
FROM (SELECT eventname,
             CAST(eventtimestamp AS bigint) - CAST(eventtimestamp AS bigint) % 3600 AS time_bucket,
             moengageuserid as hour_count,
             ROW_NUMBER() OVER (PARTITION BY eventname, moengageuserid ORDER BY eventtimestamp) as seqnum
      FROM clickstream.moengage
      WHERE date = '2020-08-20' AND
            eventname IN ('e1', 'e2', 'e3', 'e4')
    ) m
GROUP BY 1, 2, 3
ORDER BY 1, 2;