テーブル内の連続する行間の時間差を計算するにはどうすればよいですか?

How to Calculate Time Difference in Two Consecutive Table Rows

In a table with a column called "StartDate," it's often necessary to determine the time difference between adjacent records. One user recently encountered this issue while working with a requestId and startdate table and sought assistance.

The table provided by the user contained data as follows:

requestId	startdate
1	2011-10-16 13:15:56
2	2011-10-16 13:15:59
3	2011-10-16 13:15:59
4	2011-10-16 13:16:02
5	2011-10-16 13:18:07

The user's objective was to calculate the time difference between each adjacent pair of rows (e.g., requestId 1 and 2, 2 and 3, etc.). They recognized the need for a self-join but struggled to write the appropriate ON clause.

The provided solution leveraged an INNER JOIN between two instances of the same table (aliased as A and B). The ON clause specified that the requestId in row B should equal the requestId in row A plus one:

SELECT A.requestid, A.starttime, (B.starttime - A.starttime) AS timedifference
FROM MyTable A INNER JOIN MyTable B ON B.requestid = (A.requestid + 1)
ORDER BY A.requestid ASC

This query successfully calculated and returned the requested time difference between consecutive rows. However, if the requestId is not consecutive, an alternative approach is required:

SELECT A.requestid, A.starttime, (B.starttime - A.starttime) AS timedifference
FROM MyTable A CROSS JOIN MyTable B
WHERE B.requestid IN (SELECT MIN (C.requestid) FROM MyTable C WHERE C.requestid > A.requestid)
ORDER BY A.requestid ASC

In this modified query, a CROSS JOIN is used to match each row with every other row in the table. A subquery then selects the minimum requestId that is greater than the current selected requestId, ensuring that only consecutive rows are compared.

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