Optimising a SQL query which is taking a long time to execute - performance

Question

I have a simple database on which I want to do a simple query.
These are the columns for my database table:
external_id
TimeStamp
Value
Validation
Reason
Datum
Uur
Afronden
The columns are:
external_id is the id of a certain meter
Timestamp does nothing
Value is the value of that meter
Validation is just yes or no
Reason just got a varchar value with a reason
Datum is the date
Uur is the hour of that that
Afronden is a column needed for rounding
The query I want to execute got as goal to get the highest and lowest value of the sum of the value of each day.
As you see, each day got divided in hours, have to check or the datum is the same or changes and get the total value by that.
This is my query:
Declare #totaal bigInt
Declare #tussentotaal bigint
Declare #Datum varchar
Declare #datumverschil varchar
Declare #hoogste bigint
Declare #laagste bigint
Declare #teller bigint
Declare #tellettotaal bigint
set #tellettotaal = (select count(*) from cresent_opdracht_de_proost_wim.dbo.[test])
Set #teller = 1
SET #datum = (Select top(1) datum
from cresent_opdracht_de_proost_wim.dbo.[test]
order by afronden asc)
Set #datumverschil = #Datum
set #tussentotaal = 0
set #totaal = 0
set #hoogste = 1775000006856
set #laagste = 1775000006856
while #teller <= #tellettotaal
begin
if #teller = 1
Begin
set #tussentotaal = (select top(1) value
from cresent_opdracht_de_proost_wim.dbo.[test]
order by afronden asc)
if #tussentotaal != 0
begin
Set #tussentotaal = #tussentotaal/100
end
End
Else
begin
SET #tussentotaal = (Select top(1) value
from (select top (#teller) *
from cresent_opdracht_de_proost_wim.dbo.[test]) q
order by afronden desc)
Set #tussentotaal = #tussentotaal/100
end
if #tussentotaal != 0
Begin
Set #totaal = #totaal + #tussentotaal
end
SET #teller= #teller + 1
Set #datumverschil = (Select top(1) datum
from (select top (#teller) *
from cresent_opdracht_de_proost_wim.dbo.[test]) q
order by afronden desc)
if #datum != #datumverschil
Begin
if #totaal >= #hoogste
begin
set #hoogste = #totaal
end
if #totaal <= #laagste
begin
if #totaal != 0
Begin
set #laagste = #totaal
end
end
Set #datum = #datumverschil
set #totaal = 0
select #teller As teller
end
end
Select #hoogste As hoogste
Select #laagste As laagste
After after 22 minutes only 44000 rows were processed.
Does anybody know how I can optimise my query?

Answer 1

Hm, ever thought that it may be an idea not to use a broken procedural approach?
First, your logic may be broken - the time osounds too large, so maybe you just have a dead loop. Sorry, it is very hard for most peope to understand fringe langauges - i.e. anything that is not english, so you table and field names make no sense. Nonsense hardly is debuggable by people not understanting it.
THAT SAID: I think you can do with a LOT less queries in total by not running a senseless loop over top 1 - instead get everything ordered descending. Brutallly speaking: Get rid of the loop and make ONE Statement that compiles your result set. Seriously. That should be possible. Do not try to outsmart a query optimizer and go back to write procedural database code like many people did 20 years ago at dbase times. In general. your query is linear which means no parallelization and your micro managing of conditions means no query optimizer can be doind smart things (as optimization is statement by statement). Define the result set in one statement and you may wonder how efficient is it.
I am quite sure at the end you will find out that you basically try to outsmart something that is better than you writing queries. Use SQL - define the result set.
And finally, without knowing your data distribution and - ouch - indices - we can not really help. Maybe you simply miss any sensible index? Who knows - you dont tell us anything (no query plan output, no table defintiions, no index definitions).
It would possibly also have not to abuse data types: Declare #Datum varchar - OUCH. Ever heard of the DATE data type in sql server?