Querying sequences of rows in SQL

Question

Suppose I am storing events associated with users in a table as follows (with dt standing in for the timestamp of the event):

Answer 1

With Postgres 9.x this is actually quite easy:

select userid, 
       string_agg(event, '' order by dt) as event_sequence
from events
group by userid;

Using that result you can now apply a regular expression on the event_sequence:

select * 
from (
  select userid, 
         string_agg(event, '' order by dt) as event_sequence
  from events
  group by userid
) t
where event_sequence ~ 'A.*B'

With Postgres 8.x you need to find a replacement for the string_agg() function (just google for it, there are a lot of examples out there) and you need a sub-select to ensure the ordering of the aggregate as 8.x does support an order by in an aggregate function.