Question

I posted about this a few weeks ago, but I don’t think I asked the question clearly because the answers I got were not what I was looking for. I think it’s best to start again.

I’m trying to query a database to retrieve the number of unique entries over time. The data looks something like this:

Day | UserID
1 | A
1 | B
2 | B
3 | A
4 | B
4 | C
5 | D

I’d like the query result to look this this

Time Span | COUNT(DISTINCT UserID)
Day 1 to Day 1 | 2
Day 1 to Day 2 | 2
Day 1 to Day 3 | 2
Day 1 to Day 4 | 3
Day 1 to Day 5 | 4

If I do something like

SELECT COUNT(DISTINCT `UserID`) FROM `table` GROUP BY `Day`

, the distinct counts will not consider user IDs of previous days.

Any Ideas? The data set I’m using is quite large, so multiple-queries and post processing takes a long time (that’s how I’m currently doing it).

Thanks

Answer 1

You can use a subquery

Sample table

create table visits (day int, userid char(1));
insert visits values
(1,'a'),
(1,'b'),
(2,'b'),
(3,'a'),
(4,'b'),
(4,'c'),
(5,'d');

The query

select d.day, (select count(distinct userid) from visits where day<=d.day)
from (select distinct day from visits) d