Need Help in query

From: "Satish Burnwal (sburnwal)" <sburnwal(at)cisco(dot)com>
To: <pgsql-general(at)postgresql(dot)org>
Subject: Need Help in query
Date: 2010-12-23 09:37:49
Message-ID: 3A8C969225424C4D8E6BEE65ED8552DA03543C1D@XMB-BGL-41C.cisco.com
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I need a help in writing a query. I have data as follows:

mydb=# select * from usrlog ;
logid | userid | loginhr | logouthr
-------+----------+---------+----------
0 | sburnwal | 0 | 1
1 | rickyrs | 1 | 5
2 | satishbn | 1 | 6
3 | taohujin | 2 | 4
4 | jospehm | 4 | 5

Table captures the login and logout time (taking hour here to simplify)
of users and my aim to find the number of logged-in users (online users)
at every hr (1st hr i.e. 0-1, 2nd hrs i.e. 1-2, 3rd hr i.e. 2-3 and ...
so on). As the data indicates, use is not logging out in same hr as hr
of login. A user can be logged-in for more than one hr. For example,
here user rickyrs is logged-in for 1st, 2nd, 3rd, 4th and 5th hr. My
query needs to find out in the last 24 hrs, how many users were
logged-in at each hr. I want the result this way:

Nth-hr user
---------------------------
1 sburnwal
2 rickyrs
2 satishbn
3 rickyrs
3 satishbn
3 taohujin
4 rickyrs
4 satishbn
4 taohujin
4 josephm

Appreciate your response in advance. For me, even the count of users on
hourly basis is fine.

Thanks
-Satish

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