mysql arithmetik operation (subtraction) last - first of the day(week, month) - mysql

Question

i got a MySQL tbl, with some colums, where every 5 min. a new row is inserted with 3 values
1. Auto inc. curent Date Unix timestamp --> date
2. power consumption absolut --> wert01
3. Power Generation absolut --> wert02
To Show this Information in a Graph, for Exampl for weekly power consumption, i need to select the First and the last, which allready Works, but then have to Substract the last from the First and Show only tue result & the day of the werk.
SELECT
(SELECT wert01
FROM sml_splitt
WHERE date >= curdate() - INTERVAL DAYOFWEEK(curdate())+6 DAY
AND date < curdate() - INTERVAL DAYOFWEEK(curdate()) DAY
ORDER BY date DESC LIMIT 1) AS 'last',
(SELECT wert01
FROM sml_splitt
WHERE date >= curdate() - INTERVAL DAYOFWEEK(curdate())+6 DAY
AND date < curdate() - INTERVAL DAYOFWEEK(curdate()) DAY
ORDER BY date LIMIT 1) AS 'lirst
I am searching for some days to find a solution, but with no success.
Hopfuly, you could help me.

Answer 1

If you're happy with your query, then you can do the math by nesting it one more time like this: http://sqlfiddle.com/#!9/515ef/1
select t1.last, t1.first, t1.last - t1.first as result
from (
select (
select wert01
from sml_splitt
where dt >= curdate() - interval dayofweek(curdate()) + 6 day
and dt < curdate() - interval dayofweek(curdate()) day
order by dt desc limit 1
) as 'last',
(
select wert01
from sml_splitt
where dt >= curdate() - interval dayofweek(curdate()) + 6 day
and dt < curdate() - interval dayofweek(curdate()) day
order by dt limit 1
) as 'first'
) t1
;
If you really want to work with this data by week for reporting purposes, let me suggest a couple of views. The first will give you all of your distinct beginning of week dates:
create view v1 as
select date(dt) as week_begins
from sml_splitt
where dayofweek(dt) = 1
group by week_begins
The second view joins the first view with itself to give you a week beginning and week ending range:
create view v2 as
select t1.week_begins, coalesce(t2.week_begins,now()) as week_ends
from v1 t1
left join v1 t2
on t2.week_begins = t1.week_begins + interval 7 day
You can see the results here: http://sqlfiddle.com/#!9/a4d1b3/2. Notice that I'm using now() to get the current date and time if the week hasn't ended yet.
From there you can join your view with your original table and use min() and max() function with grouping to get the starting and ending 'wert' values and do any calculations on them that you like.
Here's an example: http://sqlfiddle.com/#!9/a4d1b3/6
select week_begins, week_ends,
min(wert01) as start_wert01,
max(wert01) as end_wert01,
max(wert01) - min(wert01) as power_consumed,
min(wert02) as start_wert02,
max(wert02) as end_wert02,
max(wert02) - min(wert02) as power_generated,
(max(wert02) - min(wert02)) - (max(wert01) - min(wert01)) as net_generated
from v2
inner join sml_splitt
on sml_splitt.dt >= v2.week_begins
and sml_splitt.dt < v2.week_ends
group by week_begins
I hope that helps.