aggregate functions - How to average/sum data in a day in SQL Server 2005

Question

I'm trying to average data in SQL Server 2005 in a day. Here is what my database look like this if I use simple query as

SELECT timestamp, FEED
FROM ROASTER_FEED
ORDER timestamp

Data:

timestamp               Feed    
02/07/2011 12:00:01     1246   
02/07/2011 12:00:01     1234    
02/07/2011 12:00:01     1387    
02/07/2011 12:00:02     1425   
02/07/2011 12:00:03     1263   
...    
02/07/2011 11:00:01     1153    
02/07/2011 11:00:01     1348    
02/07/2011 11:00:01     1387    
02/07/2011 11:00:02     1425    
02/07/2011 11:00:03     1223    
....   
03/07/2011 12:00:01     1226    
03/07/2011 12:00:01     1245    
03/07/2011 12:00:01     1384    
03/07/2011 12:00:02     1225    
03/07/2011 12:00:03     1363

I don't know how to average the feed when someone select a date in a month and it give display the average/sum of that day only.

For example, in the outcome, if I select day as 02/07/2011. It would give me something like this:

02/07/2011 1234 (average value/or sum)

Answer 1

One possibility, if you need to do this often enough: add three computed columns for day, month, year to your table. Those columns are computed automatically based on the timestamp column, and they're just integer values, so they're easy to use in a GROUP BY.

To do this, use these T-SQL statements:

ALTER TABLE dbo.ROASTER_FEED ADD TSDay AS DAY(timestamp) PERSISTED
ALTER TABLE dbo.ROASTER_FEED ADD TSMonth AS MONTH(timestamp) PERSISTED
ALTER TABLE dbo.ROASTER_FEED ADD TSYear AS YEAR(timestamp) PERSISTED

Now, you can easily select your data based on any day you wish:

SELECT TSDay, TSMonth, TSYear, SUM(FEED)   -- use AVG(FEED) for average values
FROM dbo.ROASTER_FEED
WHERE TSYear = 2011 AND TSMonth = 8   -- or whatever you want to grab from the table!
ORDER BY timestamp
GROUP BY TSDay, TSMonth, TSYear