Question

Suppose you have a table of the form:

create table user_activity (
    user_id int not null, 
    activity_date timestamp not null, 
    ...);

It’s easy enough to select the number of unique user_id’s in the past 30 days.

select count(distinct user_id) from user_activity where activity_date > now() - interval 30 day;

But how can you select the number of unique user_ids in the prior 30 days for each of the past 30 days? E.g. uniques for 0-30 days ago, 1-31 days ago, 2-32 days ago and so on to 30-60 days ago.

The database engine is mysql if it matters

Answer 1

You could try using a sub query:

SELECT DISTINCT `activity_date` as `day`, (
SELECT count(DISTINCT `user_id`) FROM `user_activity` WHERE `activity_date` = `day`
) as `num_uniques`
FROM `user_activity` 
WHERE `activity_date` > NOW() - INTERVAL 30 day;

This should give you the number of unique users for each day. However, I haven’t tested this since I don’t have the DB to work with.