mysql: difference between values in one column

Question

this board helped me a few times in the past.

My challange: I want to get the difference between the values within one column.

The table looks like this:

id  |  channel_id  |  timestamp   |  value
4515|    7         |1519771680000 |  7777
4518|    8         |1519772160000 |  6666
4520|    7         |1519772340000 |  8888

• id: Internal ID from Datasource. In some cases it's ordered, in other cases not. We cannot thrust this order.
• channel_id: Different data sources.
• timestamp: unix timestamp.
• value: measured value.

What I want to do:

Filter (eg channel_id = 7). Calculate the difference between one timestamp and the next one. In this example: 8888-7777

I found an solution on another database but I cannot transfer it to mysql as the windows functions are very limited. Has somebody of you an idea how to get a solution which can be used in select statements?

Thx and KR Holger

Answer 1

You can get the two rows to compare (ie subtract) by joining the table to itself:

SELECT
    a.channel_id,
    a.timestamp,
    b.timestamp,
    a.value - b.value as `difference`
FROM table a
JOIN table b
ON a.channel_id = b.channel_id and a.timestamp <> b.timestamp and a.value > b.value
GROUP BY a.channel_id
ORDER BY a.channel_id

Answer 2

You can use a "correlated subquery" for this as seen below (also see this demo) . When MySQL implements window functions such a LEAD() you could use those instead.

MySQL 5.6 Schema Setup :

CREATE TABLE Table1
    (`id` int, `channel_id` int, `timestamp` bigint, `value` int)
;

INSERT INTO Table1
    (`id`, `channel_id`, `timestamp`, `value`)
VALUES
    (4515, 7, 1519771680000, 7777),
    (4518, 8, 1519772160000, 6666),
    (4520, 7, 1519772340000, 8888)
;

Query 1 :

select
      id
    , channel_id
    , timestamp
    , value
    , nxt_value
    , nxt_value - value as diff
from (
    select
          t1.id
        , t1.channel_id
        , t1.timestamp
        , t1.value
        , (select value from table1 as t2 
           where t2.channel_id = t1.channel_id
           and t2.timestamp > t1.timestamp
           order by t2.timestamp
           limit 1) nxt_value
    from table1 as t1
    ) as d

Results :

|   id | channel_id |     timestamp | value | nxt_value |   diff |
|------|------------|---------------|-------|-----------|--------|
| 4515 |          7 | 1519771680000 |  7777 |      8888 |   1111 |
| 4518 |          8 | 1519772160000 |  6666 |    (null) | (null) |
| 4520 |          7 | 1519772340000 |  8888 |    (null) | (null) |

Answer 3

Starting from MySQL 8, you can use window functions , in case of which your query would look like this:

SELECT
  id, channel_id, timestamp, value,
  value - LAG(value, 1, 0) OVER (PARTITION BY channel_id ORDER BY timestamp) difference
FROM my_table

Answer 4

thanks for all your support. I tried a lot and created "my" solution based on a stored procedure. It is not as performant as it could be but it delivers the required values.

The code is running in a loop with a max size of repetitions in the script execution to avoid an endless step :)

#Auswahl größer CH10-Wert
set @var_max_ch10vz =
    (
    select max(data.timestamp)
    from volkszaehler.data 
    where data.channel_id=10
    )
;
#Auswahl kleinster offener Wert aus SBFSPOT
set @var_min_sbfspot = 
    (
    select min(data.timestamp_unix*1000)
    from sbfspot_u.data
    where 
        data.timestamp_vzjoin is null 
        and data.timestamp_unix >1522096327
        and data.timestamp_unix*1000 < @var_max_ch10vz
    )
;
#Abgleich gegen VZ von unten
set @var_max_vz =
    (
    select min(data.timestamp)
    from volkszaehler.data 
    where data.channel_id=10 and data.timestamp >= @var_min_sbfspot
    )
;
#Abgleich gegen VZ von oben
set @var_min_vz =
    (
    select max(data.timestamp)
    from volkszaehler.data 
    where data.channel_id=10 and data.timestamp <= @var_min_sbfspot
    )
;
#Auswahl join Zeitstempel
set @vz_join_timestamp =
    (
        select tmp.uxtimestamp 
        from (
            select @var_max_vz as uxtimestamp, abs(@var_min_sbfspot-@var_max_vz) as diff
            UNION
            select @var_min_vz as uxtimestamp, abs(@var_min_sbfspot-@var_min_vz) as diff
            ) tmp
        order by tmp.diff asc
        limit 1
    )
;