mysql: difference between values in one column - mysql

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 (e.g. 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
)
;