SELECT NVL(Min(t1.Accnum), 0) + 1
INTO newAccnum
FROM T1 t1
WHERE (t1.accnum + 1) NOT IN (SELECT t2.accnum FROM T2 t2)
AND t1.accnum > 0
AND EXISTS (SELECT t3.accnum FROM T3 t3 WHERE t3.accnum = 1);
原文：http://www.cnblogs.com/wqswjx/p/5454721.html