Select first missing id above 0

Question

I have in my columns (ID) values

                           5
                           6
                           9

I want to select first missing IDfrom above 0. My desire select value will be 1.(if 1 exists then it will selects 2 and so on...). I'm using this code:

SELECT MIN(id) As MinMissingId FROM table1 where id>=0

But my result is first existing ID and not missing

Answer 1

This will return the next unused id starting with 1, works in all cases, e.g. table is empty or there's no gap:

WITH cte AS
 (
   SELECT id FROM tab
   UNION ALL 
   SELECT 0
 )
SELECT MIN(id) + 1
FROM cte
WHERE NOT EXISTS
 (
   SELECT * 
   FROM tab 
   WHERE tab.id = cte.id + 1
 )