SELECTMIN(COMP.NEXT_SEQ_NO) NEXT_SEQ_NO
FROM tablename ORIG,
(
SELECT 1000 NEXT_SEQ_NO FROM DUALUNION ALL
SELECT SEQ_NO+1 NEXT_SEQ_NO FROM tablename
WHERE SEQ_NO >= 1000
) COMP
WHERE COMP.NEXT_SEQ_NO = ORIG.SEQ_NO(+) AND ORIG.SEQ_NO IS NULL ;
別の方法(こちらのほうが見た目は好みなのですが、上の SQL より若干コストが高いみたい)
SELECTMIN(NEXT_SEQ_NO)
FROM (
SELECT 1000 NEXT_SEQ_NO FROM DUALUNION ALL
SELECT SEQ_NO+1 NEXT_SEQ_NO FROM tablename WHERE SEQ_NO >= 1000
MINUSSELECT SEQ_NO NEXT_SEQ_NO FROM tablename WHERE SEQ_NO >= 1000
);