Question

I’m working with a logfile that records data every 10 minutes. I’m trying to come up with a query that can verify that data was in fact saved every 10 minutes.

Here are some sample timestamps:

2008-01-01 00:00:00
2008-01-01 00:10:00
2008-01-01 00:20:00
2008-01-01 00:30:00

Any ideas on this? I’d give some SQL if I thought it could be improved to be correct but I don’t have anything worth posting.

Answer 1

One trick is to make a virtual table to which you can attempt to join the the data from your logfile. In my example I’ve used the postgres generate_series function to generate a series of second values to append to an initial timestamp (I assume there is a similar function in MySQL?).

The trick is to use the virtual table to which to do a left join to the actual data, to find where there is a missing value in the logging table (i.e., where logger.timestamp will be NULL).

Something along these lines will show you where there is a missing timestamp if any.

SELECT
    y.c
    , logger.timestamp
FROM
    (SELECT 
        a +  cast(b || ' sec' as interval) as c
    FROM 
        (SELECT
               cast('2011-10-31 10:00:00' as timestamp) as a
               ,t.b from generate_series(0,100,10) as t(b)
         )x
    ) y 
    LEFT JOIN (
         SELECT timestamp from log
    ) logger ON y.c = logger.timestamp
WHERE
     logger.timestamp IS NULL;