Average the value based on day in SQL

Question

I have a table with two fields (SampleTime, value) the table likes:
SampleTime field2
1999-01-01 01:00:00 0.01
1999-01-01 02:00:00 0.01
1999-01-01 03:00:00 0.03
The sample time has one hour interval. How can get average value of field2 based on day? like:
1999-01-01 average of all values within 1/1/99
1999-01-02 average of all values within 1/2/99
1999-01-03  average of all values within 1/3/99
Is there a build-in function in SQL to implement this?  It should be very sample. But I am not sure how to write this SQL query statement. Thanks. 

Answer 1

Can explain the parameters : varchar(8), sampletime, 10?
What do these parameters mean?
Thanks.
you wanted the date in mm/dd/yy format... convert(varchar(8),sampletime,10) gets you the output in that format..
you did not mention your sql version, if sql 2008 and above, you can use date funtion(it the format does not matter).
declare @table table (SampleTime datetime,field2 decimal(3,2))
insert into @table
values
('1999-01-01 01:00:00','0.01'),
('1999-01-01 02:00:00','0.01'),
('1999-01-01 03:00:00','0.03')
select avg(Field2),Convert(varchar(8),sampletime,10) from @table group by Convert(varchar(8),sampletime,10)
--SQL 2008 and above
select avg(Field2),Cast(sampletime as date) from @table group by Cast(sampletime as date)
Hope it Helps!!