Check this out:

SELECT *
FROM sga_equiv_resol z
WHERE z.nro_resolucion = 'RCS 0102/2004';
-- 1 row (PK nro_resolucion)

SELECT *
FROM sga_equiv_operac x
WHERE x.nro_resolucion = 'RCS 0102/2004';
-- 4 rows (FK nro_resolucion)

SELECT *
FROM sga_eqdef_matrices y
WHERE y.nro_resolucion = 'RCS 0102/2004';
-- 0 rows (FK nro_resolucion)

SELECT *
FROM sga_equiv_operac x
WHERE NOT EXISTS (
SELECT DISTINCT y.nro_resolucion
FROM sga_eqdef_matrices y
WHERE x.nro_resolucion=y.nro_resolucion
);
-- 4101 rows

SELECT *
FROM sga_equiv_operac x
LEFT JOIN sga_eqdef_matrices y
ON x.nro_resolucion=y.nro_resolucion
WHERE y.nro_resolucion IS NULL;
-- 4101 rows

SELECT *
FROM sga_equiv_operac x
WHERE x.nro_resolucion NOT IN (
SELECT DISTINCT y.nro_resolucion
FROM sga_eqdef_matrices y
);
-- 0 rows


I'm using IDS 11.5
Some known bug?