Nemám kde vyzkoušet, ale něco takového by mohlo fungovat.
SELECT DISTINCT t1.kod, t1.text
FROM tabulka t1
WHERE t1.kod < 0
AND NOT EXISTS (
SELECT 1 FROM tabulka t2
WHERE t2.kod = -t1.kod
);
Nemám kde vyzkoušet, ale něco takového by mohlo fungovat.
SELECT DISTINCT t1.kod, t1.text
FROM tabulka t1
WHERE t1.kod < 0
AND NOT EXISTS (
SELECT 1 FROM tabulka t2
WHERE t2.kod = -t1.kod
);