Question

Given a table

custid | date       | action
1      | 2011-04-01 | activate
1      | 2011-04-10 | deactivate
1      | 2011-05-02 | activate
2      | 2011-04-01 | activate
3      | 2011-03-01 | activate
3      | 2011-04-01 | deactivate

The database is PostgreSQL.

I want an SQL query to show customers that were active at any stage during May.

So, in the above, that would be 1 and 2.

I just can’t get my head around the way to approach this. Any pointers?

update

Customer 2 was active during May, as he was activated Before May, and not Deactivated since he was Activated. As in, I’m alive this Month, but wasn’t born this month, and I’ve not died.

select distinct custid
from MyTable
where action = 'active' and date >= '20110501' and date < '20110601'

This approach won’t work, as it only shows activations during may, not ‘actives’.

Answer 1

Try this

select t2.custid from
(
-- select the most recent entry for each customer
select custid, date, action 
from cust_table t1 
where date = (select max(date) 
    from cust_table where custid = t1.custid)
) as t2
where t2.date < '2011-06-01'
-- where the most recent entry is in May or is an activate entry
-- assumes they have to have an activate entry before they get a deactivate entry 
and (date > '2011-05-01' or [action] = 'activate')