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Editorial Team
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Asked: 2026-06-11T09:58:17+00:00

I really don’t know how to put a query for this task. I need

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I really don’t know how to put a query for this task.

I need to get a time for the first row of each record in the table but every time when I have a group of same results I need to get the first one when it occurs and when signal is equal to 1.

I don’t know how to explain it correctly but I will make an example and you’ll see what I mean.

Records:

ID| Time | Serial | Signal
 1| 10:59|   12   |   1
 2| 11:00|   12   |   1
 3| 11:01|   12   |   1
 4| 11:01|   13   |   1
 5| 11:02|   12   |   0
 6| 11:03|   12   |   0
 7| 11:05|   13   |   1
 8| 11:07|   12   |   1

So I need this result:

ID| Time | Serial | Signal
 1| 10:59|   12   |   1
 4| 11:01|   13   |   1
 7| 11:05|   13   |   1
 8| 11:07|   12   |   1
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1 Answer

  1. Editorial Team

    Your exact logic is not clear, from what I can gather you are looking to get each row where signal is 1 where the serial value changes from one time to another.

    This can be done using a variable to track the value of the serial in the previous row (variable @t in my query below).

    SELECT  ID, Time, `Serial`, `Signal`
FROM    (   SELECT  ID,
                    Time,
                    `Serial`,
                    `Signal`,
                    CASE WHEN `Serial` = @t THEN 0 ELSE 1 END AS IsNew,
                    @t:= `Serial`
            FROM    T,
                    (SELECT @t:=0) t2
            ORDER BY Time
        ) t
WHERE   IsNew = 1
AND     `Signal` = 1

    SQL Fiddle

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