Question

I’m pretty new to database programming, and I’m in need some help writing what seems to be a complicated t-sql query.

Our database stores time data along with various levels recorded at that time. A typical measurement will last for weeks or months. I need to write a query in SqlServer2005 to identify gaps in the time data to know when the instrument wasn’t communicating. Our sample interval can be either 1 sec or .5 sec, but it will always be the same in a given measurement (ie: all samples in a measurement will be either 1 or .5 samples/sec).

Ideally I would like to get a list of
[block1-start block1-end]
[block2-start block2-end]
etc

where each block is a unit of consecutive times within the overall measurement.

Are there any commands in TSQL that would make this query easier?

Answer 1

Check this out. Since you don’t have overlapping intervals you can write a simple SQL query to return these results. The SQL below creates a dummy table variable called @Events to simulate your measurements table. The final query outputs the gaps greater than 1 second (configurable via variable @MaxIntervalAllowedBetweenEvents).

-- table with dummy data
declare @Events table (
    ID          int IDENTITY NOT NULL,
    StartDate   datetime NOT NULL,
    EndDate     datetime NOT NULL
)
INSERT @Events VALUES ('1/1/2011 1:00am', '1/1/2011 2:00am')
INSERT @Events VALUES ('1/1/2011 2:00am', '1/1/2011 3:00am')  -- no gap after previous event
INSERT @Events VALUES ('1/1/2011 3:01am', '1/1/2011 4:00am')  -- 1 minute gap
INSERT @Events VALUES ('1/1/2011 4:30am', '1/1/2011 5:00am')  -- 30 minute gap


-- this variable defines the maximum interval allowed between events
declare @MaxIntervalAllowedBetweenEvents int
set @MaxIntervalAllowedBetweenEvents = 1    -- # seconds


-- select the gaps between events
SELECT
    e1.EndDate,
    Min(e2.StartDate) as NextEventStartDate,
    DateDiff(s, e1.EndDate, Min(e2.StartDate)) as SecondsBetweenEvents
FROM
    @Events as e1
join
    -- for each event in e1, get the event that immediately follows it
    @Events as e2
        on  (e1.EndDate <= e2.StartDate)
GROUP BY
    e1.EndDate
HAVING
    -- filter out events that are too close to each other
    (DateDiff(s, e1.EndDate, Min(e2.StartDate)) > @MaxIntervalAllowedBetweenEvents)
ORDER BY
    e1.EndDate