I solved the first exercise on Daily active users, and then I also solved the second part, where it is required to take into account the platform too. My concern is that, if I look at the results from both queries, it looks like the number of daily users is different. For example, with the first query, on 2015-08-04 we have 99 users, but with the second query, for that day, we have 27+42+36=105 users. What am I missing?
Below the queries that I used.
select date(created_at), count(distinct user_id) as dau from gameplays group by 1 order by 1; select date(created_at), platform, count(distinct user_id) as dau from gameplays group by 1,2 order by 1,2;