I need a query to count the number of user instances for each distinct timestamp in SQL Server 2005

Question

Here is my data table:

userid  timestamp
------------------------
user1   2017-08-15 17:00
user1   2017-08-15 17:00
user1   2017-08-15 17:00
user2   2017-08-15 17:00
user2   2017-08-15 17:00
user3   2017-08-15 17:00
user1   2017-08-15 18:00
user1   2017-08-15 18:00
user2   2017-08-15 18:00
user2   2017-08-15 18:00
user2   2017-08-15 18:00
user3   2017-08-15 18:00

I want the result to look like this:

userid    countoftimestamp
-----------------------------
user1     3_2017-08-15 17:00
user2     2_2017-08-15 17:00
user3     1_2017-08-15 17:00
user1     2_2017-08-15 18:00

and so on.

Answer 1

SELECT userid, timestamp, count(*)
FROM datatabable
GROUP BY  userid, timestamp

Answer 2

You may notice that I used varchar(16) to truncate your timestamp down to minutes (excluding seconds and milliseconds)

Example

Select userid
      ,countoftimestamp = cast(sum(1) as varchar(25))+'_'+convert(varchar(16),timestamp,20)
 From  YourTable
 Group By userid,convert(varchar(16),timestamp,20)

Returns