SQL query to group by day and calculate elapsed time (aggregating rows) - sql

Question

I'm using SQL Server 2008 and looking how to create such a query:
I have a table:
Username nvarchar(50), LogDate datetime
Test1, 2012.01.01 00:00:00
Test2, 2012.01.01 00:00:02
Test1, 2012.01.01 00:00:05
Test3, 2012.01.01 00:00:06
Test1, 2012.01.02 00:01:01
Test2, 2012.01.02 00:02:50
Test1, 2012.01.02 00:01:01
Every few seconds users send updates to insert current datetime with his username in to the log table. I need to calculate how much hours users were "online" every day.
I assume query should sum DateDiff between two rows with the same username grouped by day.
I tried using rank, but haven't got what I want.
Thank you for your help in advance.

Answer 1

You can extract the date part of the DateTime and group by the UserName to use DateDiff on the MIN / MAX Values:
Table and Data Setup:
create table UserLog (Username nvarchar(50), LogDate DateTime);
insert into UserLog Values('Test1', '2012-01-01 00:00:00');
insert into UserLog Values('Test2', '2012-01-01 00:00:02');
insert into UserLog Values('Test1', '2012-01-01 00:00:05');
insert into UserLog Values('Test3', '2012-01-01 00:00:06');
insert into UserLog Values('Test3', '2012-01-01 00:01:26');
insert into UserLog Values('Test3', '2012-01-01 00:03:22');
insert into UserLog Values('Test3', '2012-01-01 00:05:42');
insert into UserLog Values('Test3', '2012-01-01 00:00:06');
insert into UserLog Values('Test1', '2012-01-02 00:01:01');
insert into UserLog Values('Test2', '2012-01-02 00:02:50');
insert into UserLog Values('Test1', '2012-01-02 00:01:01');
Then you can SELECT as follows:
select UserName, CAST(LogDate AS DATE) as BusinessDay,
MIN(LogDate) as FirstLogEntry, MAX(LogDate) as LastLogEntry,
DATEDIFF(second,MIN(LogDate), MAX(LogDate)) as ElapsedSeconds
FROM UserLog
GROUP BY Username, CAST(LogDate AS DATE)
This will yield the following results and you can calculate hours from the seconds. I showed seconds based on your sample data with test3 user expanded:
UserName BusinessDay FirstLogEntry LastLogEntry ElapsedSeconds
-------- ----------- ----------------------- ----------------------- --------------
Test1 2012-01-01 2012-01-01 00:00:00.000 2012-01-01 00:00:05.000 5
Test2 2012-01-01 2012-01-01 00:00:02.000 2012-01-01 00:00:02.000 0
Test3 2012-01-01 2012-01-01 00:00:06.000 2012-01-01 00:05:42.000 336
Test1 2012-01-02 2012-01-02 00:01:01.000 2012-01-02 00:01:01.000 0
Test2 2012-01-02 2012-01-02 00:02:50.000 2012-01-02 00:02:50.000 0