Select first and last row value from 7 selected rows PHP/SQL

Question

I'm trying to select the 1st and last row(row 7) from 7 rows pulled from MySQL table. Looking at nesting a query is really confusing me to be honest. Using >

select location, value1, reading_time from sensordata
where id in 
(
    SELECT min(id)
    FROM sensordata
    WHERE id >= 0
    and date(`reading_time`) >= '2013-04-01'
    group by date(`reading_time`)
) 
ORDER BY `sensordata`.`reading_time` DESC 
LIMIT 7

This is working fine to pull the 7 days data I need(i want to use it to pull the last 7 days whenever the code is run) and displays the info I need to work with. But, I then want to subtract the last(most recent) entry from the earlier entry(row 7) to show usage over the last 7 days. This value, from column 'value1', is the only info I need and would like to display this, either by assigning a string to it or some other means.

When running the query i get this response in phpmyadmin(sorry its in text)>

location ..value1 ...reading_time

Kero Tank 802.46 2021-07-02 00:28:17   <<<<<<<< A
Kero Tank 802.46 2021-07-01 00:28:17
Kero Tank 802.46 2021-06-30 00:28:17
Kero Tank 811.48 2021-06-29 00:28:18
Kero Tank 811.48 2021-06-28 00:28:17
Kero Tank 811.48 2021-06-27 00:27:51
Kero Tank 811.48 2021-06-26 00:27:47   <<<<<<<< B

I want the result of BA for value1

using PHPMyAdmin

Answer 1

I am thinking window functions:

select distinct id, value1_last - value1_first
from (select sd.*,
             first_value(value1) over (partition by id order by reading_time) as value1_first,
             first_value(value1) over (partition by id order by reading_time desc) as value1_last
      from sensor_data sd
      where sd.id <> 0 and
            sd.reading_time >= '2013-04-01'
     ) sd;

Note: The subquery is not strictly necessary. I wrote it that way because I suspect there might be other columns that you want as well, so you really want aggregation rather than select distinct .