Oracle 查找未用 编码

Select Min(N.编码) As Max_编码
From (
       Select Min(M.比较码) As 编码, 'N' As 类别
       From (
              Select Lpad(Level, 2, '0') As 比较码
              From dual
              Connect By Level <= (Select Max(A.编码) From 用户表 A)
                     Minus
              Select A.编码 As 比较码 From 用户表 A
              ) M
Union All
Select Lpad(Nvl(Max(编码) + 1, '1'), 2, '0') As 编码, 'Y' As 类别 From 用户表
) N