How to find gaps in a sequence of numbers in SQL Server

Question

I am trying to find the smallest missing number in this table.

+------+--------+
| id   | number |
+------+--------+
|  902 |    1   |
|  908 |    2   |
| 1007 |    7   |
| 1189 |    8   |
| 1233 |   12   |
| 1757 |   15   |
+------+--------+

In the number column, you can see there are several gaps between the numbers. I need to get the number after the smallest gap. So in the case above I need the number 3. Because 2 is the smallest number after a gap.

Answer 1

I would use lead() :

select min(id) + 1
from (select t.*,
             lead(id) over (order by id) as next_id
      from t
     ) t
where next_id <> id + 1;

If you want to ensure that the ids start at 1 (so if "1" is missing, then return that), you can do:

select (case when min(minid) <> 1 then 1 else min(id) + 1 end)
from (select t.*, min(id) over () as minid
             lead(id) over (order by id) as next_id
      from t
     ) t
where next_id <> id + 1;

Answer 2

Lots of different ways here is one that is probably popular these days due to using window functions.

;WITH cte AS (
    SELECT
       Id
       ,[Number]
       ,LAG([Number],1,NULL) OVER (ORDER BY [Number] ASC) AS LagValue
    FROM
       @Table
)

SELECT
    MIN(LagValue) + 1 AS SmallestNumberAfterGap
FROM
    cte
WHERE
    LagValue <> [Number] - 1

And here is one that is less overall code using an LEFT JOIN

SELECT
    MIN(t2.[Number]) + 1 AS SmallestNumberAfterGap
FROM
    @Table t1
    LEFT JOIN @Table t2
    ON t1.Number + 1 = t2.Number

And because I am just writing more code here is one using EXISTS

SELECT
    MIN(t1.Number) + 1 AS SmallestNumberAfterGap
FROM
    @Table t1
WHERE
    NOT EXISTS (SELECT * FROM @Table t2 WHERE t1.Number + 1 = t2.Number)

Here is a link showing all 3 options functioning http://rextester.com/TIFRI87282

Answer 3

This solution will give you the minimum numbers missing from sequence not a single number .. This uses a numbers table ..

Demo Here

;With cte
as
(
select 
*
from 
Numbers n
left join
#t1
on n.Number=#t1.num
where n.Number<=(select max(num) from #t1)

)
select 
number from cte c1
join
#t1
on #t1.num+1=c1.Number
where c1.num is null