How do you find the smallest unused number in a SQL Server column?
I am about to import a large number of manually recorded records from Excel into a SQL Server table. They all have a numeric ID (called document number), but they weren't assigned sequentially for reasons that no longer apply, meaning from now on when my web site records a new record, it needs to assign it the smallest possible document number (greater than zero) that has not already been taken.
Is there a way to do this through plain SQL or is this a problem for TSQL/code?
Thanks!
EDIT
Special thanks to WW for raising the issue of concurrency. Given that this is a web app, it is multi-threaded by definition and anyone faced with this same problem should consider either a code or DB level lock to prevent a conflict.
LINQ
FYI - this can be accomplished via LINQ with the following code:
var nums = new [] { 1,2,3,4,6,7,9,10};
int nextNewNum = (
from n in nums
where !nums.Select(nu => nu).Contains(n + 1)
orderby n
select n + 1
).First();
nextNewNum == 5
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!
declare @value int
select @value = case
when @value is null or @value + 1 = idcolumn
then idcolumn
else @value end
from table
order by idcolumn
select @value + 1
Does 1 table scan rather than 2 scans a hash match and a join like the top answer
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.
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)
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
Here is a simple approach. It may no be fast. It will not find missing numbers at the beginning.
SELECT MIN(MT1.MyInt+1)
FROM MyTable MT1
LEFT OUTER JOIN MyTable MT2 ON (MT1.MyInt+1)=MT2.MyInt
WHERE MT2.MyInt Is Null
Let's assume your IDs should always start with 1:
SELECT MIN(a.id) + 1 AS firstfree
FROM (SELECT id FROM table UNION SELECT 0) a
LEFT JOIN table b ON b.id = a.id + 1
WHERE b.id IS NULL
This handles all cases I can think of - including no existing records at all.
The only thing I don't like about this solution is that additional conditions have to be included twice, like that:
SELECT MIN(a.id) + 1 AS firstfree
FROM (SELECT id FROM table WHERE column = 4711 UNION SELECT 0) a
LEFT JOIN table b ON b.column = 4711 AND b.id = a.id + 1
WHERE b.id IS NULL
Please also notice the comments about locking and concurrency - the requirement to fill gaps is in most cases bad design and can cause problems. However, I had a good reason to do it: the IDs are to be printed and typed by humans and we don't want to have IDs with many digits after some time, while all the low ones are free...
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.
I know this answer is late but you can find the smallest unused number by using a recursive table expression:
CREATE TABLE Test
(
ID int NOT NULL
)
--Insert values here
;WITH CTE AS
(
--This is called once to get the minimum and maximum values
SELECT nMin = 1, MAX(ID) + 1 as 'nMax'
FROM Test
UNION ALL
--This is called multiple times until the condition is met
SELECT nMin + 1, nMax
FROM CTE
WHERE nMin < nMax
)
--Retrieves all the missing values in the table. Removing TOP 1 will
--list all the unused numbers up to Max + 1
SELECT TOP 1 nMin
FROM CTE
WHERE NOT EXISTS
(
SELECT ID
FROM Test
WHERE nMin = ID
)
I faced a similar problem and came up with this:
Select Top 1 IdGapCheck
From (Select Id, ROW_NUMBER() Over (Order By Id Asc) AS IdGapCheck
From dbo.table) F
Where Id > IdGapCheck
Order By Id Asc
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
来源：https://stackoverflow.com/questions/684106/find-the-smallest-unused-number-in-sql-server