T-sql; adding positive and negative values with SUM() - sql

Question

I haven´t done any T-SQL before and were wondering how could I change the script below to differentiate between positive and negative values. At the moment it seems to just to add the two together and giving out wrong values.
I would like to add an IF clause that s.summa would follow rules:
e.g
-1 + -1 = -2
1 + -1 = 0
1 + 1 = 2
If I am correct now it is being dealt with "etumerkki", so that if etumerkki variable is 0 s.sum is converted to negative and if etumerkki variable has not been set it remains normal.
SET #stmt = CONCAT( 'INSERT INTO tbl_1 (rivi,rivi_id,isanta_rivi,taso,lihavointi,tili_rivi,otsikko,selite,summa)
SELECT t.rivi , t.rivi_id , t.isanta_rivi , t.taso , t.lihavointi , t.rivi_tyyppi , t.otsikko , t.selite ,
CASE etumerkki
WHEN 0 THEN -1 * IFNULL(SUM(s.summa),0.00)
ELSE IFNULL(SUM(s.summa),0.00)
END AS summa
FROM tase_mem t
LEFT JOIN kuutio_paakirja s ON s.tili = t.tili
WHERE ' , nyk_summa_ehto , '
AND s.kpaikka_id = ' , kpaikka_id , ' AND s.projekti_id = ' , projekti_id , ' ' , isanta_ehto , '
GROUP BY t.rivi , t.rivi_id , t.taso , t.lihavointi , t.rivi_tyyppi , t.etumerkki , t.otsikko , t.selite;');
PREPARE stmt FROM #stmt;
EXECUTE stmt;
Thanks in advance

Answer 1

Not sure if i follow...
SUM()
does indeed evaluate signed numbers correctly. Eg
sum(1, -1) = 0, sum(-1, -1)=-2 and sum(1,1) = 2
See:
select sum(a)
from (
select 1 as a
union all select -1
) i
select sum(a)
from (
select -1 as a
union all select -1
) i
select sum(a)
from (
select 1 as a
union all select 1
) i
However, in your query, you write IFNULL(SUM(s.summa),0.00):
This means that your query first sums all s.summa, and AFTER that evaluates IFNULL.
I suppose you want to evalue whether s.summa is null before you add them together. changing the query to SUM(IFNULL(s.summa),0.00) should give you the proper result.