Nebo
SELECT DISTINCT t1.kod, t1.text
FROM tabulka t1
LEFT OUTER JOIN tabulka t2 on t1.kod = -t2.kod
WHERE t1.kod < 0 and t2.kod is null
Nebo
SELECT DISTINCT t1.kod, t1.text
FROM tabulka t1
LEFT OUTER JOIN tabulka t2 on t1.kod = -t2.kod
WHERE t1.kod < 0 and t2.kod is null