missing number or max number from the list - sql

Question

I am looking for missing number in the list, it works perfectly fine but when it start from 2, would it be possible to get 1. in below insertion
it should provide 1 not 4. please help thanks
drop table #temp
create table #temp
(
Number INT
)
insert into #temp
(Number)
select 2 union all
select 3 union all
select 5
SELECT MIN(t1.Number) + 1 AS MissingNumber
FROM #temp t1
LEFT OUTER JOIN #temp t2 ON (t1.Number + 1 = t2.Number)
WHERE t2.Number IS NULL

Answer 1

I will suggest you to create a separate numbers table to do this.
There a many ways to create number table. Check this link for more info
SELECT TOP (1000) n = Row_number()OVER (ORDER BY number)
INTO #numbers
FROM [master]..spt_values
ORDER BY n;
CREATE TABLE #temp
(Number INT)
INSERT INTO #temp(Number)
SELECT 2
UNION ALL
SELECT 3
UNION ALL
SELECT 5
SELECT Min(t1.n) AS MissingNumber
FROM #numbers t1
LEFT OUTER JOIN #temp t2
ON ( t1.n = t2.Number )
WHERE t2.Number IS NULL

Answer 2

I think its not possible because join doesn't knows that number starts from 1. it will search for min value.
we can use while loop to solve problem

Answer 3

use this
IF OBJECT_ID('Tempdb..#temp') IS NOT NULL
DROP TABLE #temp
CREATE TABLE #temp ( Number INT )
INSERT INTO #temp
( Number )
VALUES ( 2 ),
( 3 ),
( 5 );
WITH cte
AS ( SELECT n = 1
UNION ALL
SELECT n + 1
FROM cte
WHERE n <= 100 --can be increased with OPTION ( MAXRECURSION {iteration value} ) at the end of the query
)
SELECT MIN(cte.n) AS MissingNumber
FROM cte
LEFT JOIN #temp t ON ( cte.n = t.Number )
WHERE t.Number IS NULL