With sql find next available integer within range that is not present in existing integer subset(s)

Question

Problem statement:

given a range x -> y of unsigned integers
where x and y are both in the range

Answer 1

This is a kind of question I usually try to solve with a simple cumulative sum over +1/-1.

ip_address: ip is not available for ip_address, but available starting with ip_address + 1

subnet: ip is not available for ipv4_end, but available stating with ipv4_begin + 1

dhcp_range: ip is not available after begin_address, but available starting with end_address + 1

Now sum all the +1/-1 ordered by ip addresses, whenever it's greater than zero it's the start of a range of free tips and now the next row's ip is the start of a used range.

SELECT
   subnet_sk
  ,ip_begin
  ,ip_end
FROM
 (
   SELECT
      subnet_sk
     ,ip AS ip_begin
    -- ,x
     ,LEAD(ip)
      OVER (ORDER BY ip, x) - 1 AS ip_end
     ,SUM(x)
      OVER (ORDER BY ip, x 
            ROWS UNBOUNDED PRECEDING) AS avail
   FROM
   (
      SELECT
         subnet_sk, CAST(ipv4_begin AS BIGINT)+1 AS ip, 1 AS x 
      FROM subnet
   --   WHERE subnet_sk = 1

      UNION ALL

      SELECT
         subnet_sk, CAST(ipv4_end AS BIGINT), -1 
      FROM subnet
   --   WHERE subnet_sk = 1

      UNION ALL

      SELECT
         subnet_sk, CAST(begin_address AS BIGINT), -1
      FROM dhcp_range
   --   WHERE subnet_sk = 1

      UNION ALL

      SELECT
         subnet_sk, CAST(end_address AS BIGINT)+1, 1 
      FROM dhcp_range
   --   WHERE subnet_sk = 1

      UNION ALL

      SELECT
         subnet_sk, CAST(address AS BIGINT), -1 
      FROM ip_address
   --   WHERE subnet_sk = 1

      UNION ALL

      SELECT
         subnet_sk, CAST(address AS BIGINT)+1, 1 
      FROM ip_address
   --   WHERE subnet_sk = 1
   ) AS dt
 ) AS dt
WHERE avail > 0

This will return all available ranges, for a single subnet simply uncomment the WHERE-condition: fiddle