How can i get the first free ID from table - sql

Question

I have a table with a primary key field ID. I don't want to use Identity because i need to give the User the posibility of manualy choose an ID for the new object. So my idea is:
By default in the edit view, the ID field will be 0.
If user don't change it, i need to find the first free ID and use it.
If the user change the ID, i first need to check if there's another object with that id, and in that case throw an error.
If not, use the ID choose by user.
create the new object
The question is how to query and SQL Server table to get the firts free ID number?
Examples 1:
ID
--
1
2
10
First free ID is 3
Examples 2:
ID
--
1
2
3
4
First free ID is 5
Is there a way to do that?
All i can think of is get the min and max value, create a cycle for possible values and then compare with table data, but it involves too many querys to the database.
Thanks!

Answer 1

You can find the first free id as the first id where there is no "next" value:
select coalesce(min(t.id) + 1, 0)
from table t left outer join
table t2
on t.id = t2.id - 1
where t2.id is null;
EDIT:
If you want to handle "1" as a potential missing value:
select (case when min(minid) > 1 then 1 else coalesce(min(t.id) + 1, 0) end)
from table t left outer join
table t2
on t.id = t2.id - 1 cross join
(select min(id) as minid from table t) const
where t2.id is null;

Answer 2

Test Table
CREATE TABLE ID_TABLE(ID INT)
INSERT INTO ID_TABLE VALUES
(1),(2),(10)
Stored Procedure
ALTER PROCEDURE dbo.usp_GetNextValue
#nxt_ID_Wanted INT = 0,
#nxt_ID_Available INT OUTPUT
AS
BEGIN
SET NOCOUNT ON;
-- If user hasnt passed any value get next avilable value
IF (#nxt_ID_Wanted = 0)
BEGIN
SELECT TOP 1 #nxt_ID_Available = ID + 1
FROM
(
SELECT ID , ROW_NUMBER() OVER (ORDER BY ID ASC) AS rn
FROM ID_TABLE
)Q
WHERE ID = rn
ORDER BY ID DESC
IF (#nxt_ID_Available IS NULL)
BEGIN
SET #nxt_ID_Available = 1;
END
END
-- If user has passed a value check if it exists and raise error
ELSE IF EXISTS(SELECT 1 FROM ID_TABLE WHERE ID = #nxt_ID_Wanted)
BEGIN
RAISERROR('Selected ID value already exists',16,1)
SET #nxt_ID_Wanted = 0;
RETURN;
END
ELSE -- else just let the user have the value he/she wanted
BEGIN
SET #nxt_ID_Available = #nxt_ID_Wanted;
END
END
Execute Procedure
DECLARE #ID INT;
EXECUTE dbo.usp_GetNextValue #nxt_ID_Wanted = 6
,#nxt_ID_Available = #ID OUTPUT
SELECT #ID

Answer 3

This function gets first free ID for a table in PostgreSQL (PL/pgSQL):
CREATE OR REPLACE FUNCTION next_free_id_from(table_name TEXT)
RETURNS BIGINT AS
$$
DECLARE
next_id BIGINT;
BEGIN
execute 'select rn from (select id, row_number() over (order by id) as rn from ' || quote_ident(table_name) || ') as T where id > rn limit 1' into next_id;
if next_id is null then
execute 'select COALESCE(max(id), 0) + 1 from ' || quote_ident(table_name) into next_id;
end if;
return next_id;
END;
$$ language 'plpgsql' STRICT;
Execute:
select next_free_id_from('test');
Where test is a table name. This function call can be placed in DEFAULT of the primary key.