Question

I am trying to find an elegant solution in the form of a SQL query for the following problem.

New records will be inserted in the Log table.
I need to detect any new records (inserted in the last hour) that I haven’t seen before and generate an alert (e.g. # of these records > 0)

ID, Url, DOB
1, site1.com/page1, "5/06/2012 20:01"
2, site2.com/page2, "5/06/2012 21:20"
3, site1.com/page1, "6/06/2012 10:05"

If “now” is 6/06/2012 10:40 – I see that there was 1 new record (id=3) inserted but I don’t want to generate an alert because we have seen this URL before (id=1).

if we have
4, site3.com/pageX, “6/06/2012 10:08”
then I want to generate an alert (return count=1) because this row was inserted in the last hour and we haven’t seen it before.

What is the best way to implement it? ideally without nested queries

Answer 1

I think this is what you are after. This will retrieve new entries in the last hour (Where new means the same URL has not been visited unitl the last hour)

SELECT  *
FROM    Log
WHERE   DOB > DATEADD(HOUR, -1, CURRENT_TIMESTAMP)
AND     NOT EXISTS
        (   SELECT  1
            FROM    Log T1
            WHERE   T1.URL = Log.URL 
            AND     T1.DOB < DATEADD(HOUR, -1, CURRENT_TIMESTAMP)
        )

Working example on SQL Fiddle

EDIT

Just seen a comment that you only need a count:

SELECT  COUNT(*)
FROM    Log
WHERE   DOB > DATEADD(HOUR, -1, CURRENT_TIMESTAMP)
AND     NOT EXISTS
        (   SELECT  1
            FROM    Log T1
            WHERE   T1.URL = Log.URL 
            AND     T1.DOB < DATEADD(HOUR, -1, CURRENT_TIMESTAMP)
        )

EDIT 2

I am not sure why there is the requirement of only being a single select, however, the closest I can get to a single select is this:

SELECT  COUNT(*)
FROM    (   SELECT  *, MIN(DOB) OVER(PARTITION BY URL) [FirstViewed]
            FROM    Log
        ) Log
WHERE   FirstViewed >= DATEADD(HOUR, -1, CURRENT_TIMESTAMP)

This will still return 2 if the same page has been visited twice in the last hour.

http://sqlfiddle.com/#!3/5a8bc/1