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    <p>You could probably have it much more simplified by implementing @ mysql variables. Pre-query to get an ID, and the ID of the next associated with it, then do a join to the original table TWICE (different aliases) to have proper values, then do whatever calc you need... So, the "LastID" column is whatever the @WasLastID value was IF it was within the same skill target ID qualification. If not, it gets set back to zero. AFTER THAT TEST is applied, then the @WasLastID gets the ID of the current record as BASIS of the next entry's test.</p> <pre><code>select t.id, t.SkillTargetID, t.UTC_DateTime, t.UTC_DateTime2, if( @WasLastTarget = t.SkillTargetID, @WasLastID, 0 ) LastID, @WasLastID := t.ID as tmpLastID, @WasLastTarget := t.SkillTargetID as tmpLastTarget from testdur t, ( select @WasLastID = 0, @WasLastTarget := 0 ) sqlvars order by t.skillTargetID, t.id </code></pre> <p>The order by will be applied to the result set before any of the @ variables are processed, so the above query would in essence create the following result ensuring all skill targets are properly sequenced when applying the @ last target comparisons...</p> <pre><code> +----+---------------+--------------+--------+------------+---------------+ | id | SkillTargetID | UTC_DateTime | LastID | tmpLastID | tmpLastTarget | +----+---------------+--------------+--------+------------+---------------+ | 1 | 5000 | 1323719341 | 0 | 1 | 5000 | 3 | 5000 | 1323719342 | 1 | 3 | 5000 | 5 | 5000 | 1323719343 | 3 | 5 | 5000 | 10 | 5000 | 1323719445 | 5 | 10 | 5000 | 13 | 5000 | 1323719445 | 10 | 13 | 5000 | 16 | 5000 | 1323719460 | 13 | 16 | 5000 | 17 | 5000 | 1323719460 | 16 | 17 | 5000 -- BREAK BETWEEN SKILL TARGET... | 2 | 5010 | 1323719341 | 0 | 2 | 5010 | 4 | 5010 | 1323719342 | 2 | 4 | 5010 | 7 | 5010 | 1323719350 | 4 | 7 | 5010 | 8 | 5010 | 1323719441 | 7 | 8 | 5010 | 9 | 5010 | 1323719444 | 8 | 9 | 5010 | 14 | 5010 | 1323719445 | 9 | 14 | 5010 -- BREAK BETWEEN SKILL TARGET... | 6 | 5055 | 1323719345 | 0 | 6 | 5055 | 11 | 5055 | 1323719445 | 6 | 11 | 5055 -- BREAK BETWEEN SKILL TARGET... | 12 | 5060 | 1323719445 | 0 | 12 | 5060 | 15 | 5060 | 1323719446 | 12 | 15 | 5060 | 18 | 5060 | 1323719500 | 15 | 18 | 5060 +----+---------------+--------------+--------+------------+---------------+ </code></pre> <p>Now, all that said and sample provided, you could then expand on it such as...</p> <pre><code>select PreQuery.*, CASE WHEN t1.UTC_DateTime IS NULL THEN 0 ELSE t2.UTC_DateTime - t1.UTC_DateTime END) AS Duration from ( above full select query ) as PreQuery LEFT JOIN testdur t1 on PreQuery.ID = t1.ID LEFT JOIN testdur t2 on PreQuery.LastID = t2.ID </code></pre>
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    1. COI ended doing most of this in code instead of SQL, but your idea does work - I tested it. I will probably be able to use this idea for other things though. Thanks for taking the time.
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