Find the smallest unused number in SQL Server

Find the first row where there does not exist a row with Id + 1

SELECT TOP 1 t1.Id+1 
FROM table t1
WHERE NOT EXISTS(SELECT * FROM table t2 WHERE t2.Id = t1.Id + 1)
ORDER BY t1.Id

Edit:

To handle the special case where the lowest existing id is not 1, here is a ugly solution:

SELECT TOP 1 * FROM (
    SELECT t1.Id+1 AS Id
    FROM table t1
    WHERE NOT EXISTS(SELECT * FROM table t2 WHERE t2.Id = t1.Id + 1 )
    UNION 
    SELECT 1 AS Id
    WHERE NOT EXISTS (SELECT * FROM table t3 WHERE t3.Id = 1)) ot
ORDER BY 1

If you sort them by numeric ID, the number you are looking for will be the first one for which the ROW_NUMBER() function doesn't equal the ID.

No mention of locking or concurrency in any of the answers so far.

Consider these two users adding a document at nearly the same time:-

User 1                User 2
Find Id               
                      Find Id
Id = 42               
                      Id = 42
Insert (42..)  
                      Insert (42..)
                      Error!

You either need to: a) Handle that error and go around the loop again looking for the next available Id, OR b) Take a lock out at the start of the process so only 1 user is looking for Ids at a particular time

SELECT TOP 1 t1.id+1
FROM mytable t1
 LEFT OUTER JOIN mytable t2 ON (t1.id + 1 = t2.id)
WHERE t2.id IS NULL
ORDER BY t1.id;

This is an alternative to the answers using correlated subqueries given by @Jeffrey Hantlin and @Darrel Miller.

However, the policy you're describing is really not a good idea. ID values should be unique, but should not be required to be consecutive.

What happens if you email someone with a link to document #42, and then subsequently delete the document? Later, you re-use the id #42 for a new document. Now the recipient of the email will follow the link to the wrong document!

Is there a reason that it has to be the smallest possible number? Why do you need to fill the holes?

Edit to ad the answer, since it's a business rule.

DECLARE @counter int
DECLARE @max
SET @counter = 0
SET @max = SELECT MAX(Id) FROM YourTable
WHILE @counter <= @max
BEGIN
    SET @counter = @counter + 1
    IF NOT EXISTS (SELECT Id FROM YourTable WHERE Id = @counter)
        BREAK
    END
END

(I don't have a db handy, so this may not be 100% accurate, but you should be able to get it from there)

If there are gaps in the sequence, you can find the first gap with something like this:

select top 1 (found.id + 1) nextid from (select id from items union select 0) found
    where not exists (select * from items blocking
                          where blocking.id = found.id + 1)
    order by nextid asc

In other words, find the least ID whose successor does not exist, and return that successor. If there are no gaps, it returns one greater than the greatest extant ID. A placeholder ID of 0 is inserted to insure that IDs starting with 1 are considered.

Note that this will take at least n log n time.

Microsoft SQL permits the use of a from clause in an insert statement, so you may not need to resort to procedural code.

select
    MIN(NextID) NextUsableID
from (
    select (case when c1 = c2 then 0 
            else c1 end) NextID 
    from (  select ROW_NUMBER() over (order by record_id) c1, 
                   record_id c2
            from   myTable)
)
where NextID > 0

You really should try to convert the column to IDENTITY. BACKUP first then use ROW_NUMBER to update the document ID so they start from 1 and up to the document count. You should do it in a WHILE one at the time because if the number column is used as reference in other tables (foreign keys) SQL Server will try to update the foreign keys and maybe fail because of conflicts. In the end just enable identity specifications for the column.

:) It's more work now but it will save you a lot of trouble later.

For Oracle DB this should do the Job:

SELECT MIN(NI) FROM
        (SELECT ROWNUM AS NI,YOUR_ID
         FROM (SELECT YOUR_ID
               FROM YOUR_TABLE 
               ORDER BY YOUR_ID ASC))
WHERE NI<>YOUR_ID

ROW_NUMBER() function example:

IF NOT EXISTS (SELECT TOP 1 row_num FROM (SELECT ROW_NUMBER() OVER (ORDER BY Id) row_num, Id FROM table) t WHERE t.Id > t.row_num) SELECT MAX (Id)+1 FROM table ELSE SELECT TOP 1 row_num FROM (SELECT ROW_NUMBER() OVER (ORDER BY Id) row_num, Id FROM table) t WHERE t.Id > t.row_num;

For those who are really care about the "smallest" unused number, i.e., must be smallest, then you can take a look at https://stackoverflow.com/a/72758891/2293666, I have converted it to SQLSever syntax:

(Assuming the minimum number is 1)

from (
    select
        sequence,
        case
        when sequence <> IsNull(lag(sequence) over (order by sequence), 0) + 1      -- when seq <> prev_seq_add_1
            then IsNull(lag(sequence) over (order by sequence), 0) + 1              -- then prev_seq_add_1
        when sequence <> IsNull(lead(sequence) over (order by sequence), 0) - 1     -- when seq <> next_seq_dec_1
            then sequence + 1                                                       -- then seq + 1
        end
        as unused_seq
    from (
        select 3 as sequence
        union select 2 as sequence
        union select 5 as sequence
        union select 1 as sequence
        union select 7 as sequence
    ) as unique_sequences
) as t
where unused_seq is not null
    order by sequence

Please replace the unique_sequences subquery with yours.

You can paste the sql to sqlfiddle to have a test without installing anything. sqlfiddle: http://sqlfiddle.com/#!18/9eecb/164624

Tested results:

• 1,2,3,4,5 -> 6
• 3,4,5,7 -> 1
• 1,2,4,5 -> 3

Note that No Record means 1.

The query could be further simplified if you can find some inline sequence table then it would be like that in mysql

SELECT * FROM seq_1_to_32767
   EXCEPT
   SELECT sequence from .... order by sequence.

but I could not find appropriate one.