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You'd have to store the last allocated identifier in the sequence.
For example, store it in another table that has one column & one row.
CREATE TABLE CurrentMaxId (
Id CHAR(6) NOT NULL
);
INSERT INTO CurrentMaxId (Id) VALUES ('AAAAAA');
Each time you allocate a new identifier, you'd fetch the value in that tiny table, increment it, and store that value in your main table as well as updating the value in CurrentMaxId.
The usual caveats apply with respect to concurrency, table-locking, etc.
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I think I'd have tried to store the sequence as an integer, then translate it to string. Or else store a parallel integer column that is incremented at the same time as the alpha value. Either way, you could sort on the integer column.
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A problem here is that you can't really tell from the data where the "last" entry is unless there is more detail as to how the old entries are deleted.
If I understand correctly, you are wrapping around at the end of the sequence, which means you must be deleting some of your old data to make space. However if the data isn't deleted in a perfectly uniform manner, you'll end up with fragments, like below:
ABCD HIJKL NOPQRS WXYZ
You'll notice that there is no obvious next value...D could be the last value created, but it might also be L or S.
At best you could look for the first or last missing element (use a stored procedure to perform a x+1 check just like you would to find a missing element in an integer sequence), but it's not going to provide any special result for rolled-over lists.
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Since I don't feel like writing code to increment letters, I'd create a table of all valid IDs (AAAAAA through ZZZZZZ) with an integer from 1 to X for those IDs. Then you can use the following:
SELECT @max_id = MAX(id) FROM Possible_Silly_IDs
SELECT
COALESCE(MAX(PSI2.silly_id), 'AAAAAA')
FROM
My_Table T1
INNER JOIN Possible_Silly_IDs PSI1 ON
PSI1.silly_id = T1.silly_id
INNER JOIN Possible_Silly_IDs PSI2 ON
PSI2.id = CASE WHEN PSI1.id = @max_id THEN 1 ELSE PSI1.id + 1 END
LEFT OUTER JOIN My_Table T2 ON
T2.silly_id = PSI2.silly_id
WHERE
T2.silly_id IS NULL
The COALESCE is there in case the table is empty. To be truly robust you should calculate the 'AAAAAA' (SELECT @min_silly_id = silly_id WHERE id = 1) in case your "numbering" algorithm changes.
If you really wanted to do things right, you'd redo the database design as has been suggested.
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Here's code that I think will give you your Next value. I created 3 functions. The table is just my simulation of the table.column with your alpha ids (I used MyTable.AlphaID). I assume that it's as you implied and there is one contiguous block of five-character uppercase alphabetic strings (AlphaID):
IF OBJECT_ID('dbo.MyTable','U') IS NOT NULL
DROP TABLE dbo.MyTable
GO
CREATE TABLE dbo.MyTable (AlphaID char(5) PRIMARY KEY)
GO
-- Play with different population scenarios for testing
INSERT dbo.MyTable VALUES ('ZZZZY')
INSERT dbo.MyTable VALUES ('ZZZZZ')
INSERT dbo.MyTable VALUES ('AAAAA')
INSERT dbo.MyTable VALUES ('AAAAB')
GO
IF OBJECT_ID('dbo.ConvertAlphaIDToInt','FN') IS NOT NULL
DROP FUNCTION dbo.ConvertAlphaIDToInt
GO
CREATE FUNCTION dbo.ConvertAlphaIDToInt (@AlphaID char(5))
RETURNS int
AS
BEGIN
RETURN 1+ ASCII(SUBSTRING(@AlphaID,5,1))-65
+ ((ASCII(SUBSTRING(@AlphaID,4,1))-65) * 26)
+ ((ASCII(SUBSTRING(@AlphaID,3,1))-65) * POWER(26,2))
+ ((ASCII(SUBSTRING(@AlphaID,2,1))-65) * POWER(26,3))
+ ((ASCII(SUBSTRING(@AlphaID,1,1))-65) * POWER(26,4))
END
GO
IF OBJECT_ID('dbo.ConvertIntToAlphaID','FN') IS NOT NULL
DROP FUNCTION dbo.ConvertIntToAlphaID
GO
CREATE FUNCTION dbo.ConvertIntToAlphaID (@ID int)
RETURNS char(5)
AS
BEGIN
RETURN CHAR((@ID-1) / POWER(26,4) + 65)
+ CHAR ((@ID-1) % POWER(26,4) / POWER(26,3) + 65)
+ CHAR ((@ID-1) % POWER(26,3) / POWER(26,2) + 65)
+ CHAR ((@ID-1) % POWER(26,2) / 26 + 65)
+ CHAR ((@ID-1) % 26 + 65)
END
GO
IF OBJECT_ID('dbo.GetNextAlphaID','FN') IS NOT NULL
DROP FUNCTION dbo.GetNextAlphaID
GO
CREATE FUNCTION dbo.GetNextAlphaID ()
RETURNS char(5)
AS
BEGIN
DECLARE @MaxID char(5), @ReturnVal char(5)
SELECT @MaxID = MAX(AlphaID) FROM dbo.MyTable
IF @MaxID < 'ZZZZZ'
RETURN dbo.ConvertIntToAlphaID(dbo.ConvertAlphaIDToInt(@MaxID)+1)
IF @MaxID IS NULL
RETURN 'AAAAA'
SELECT @MaxID = MAX(AlphaID)
FROM dbo.MyTable
WHERE AlphaID < dbo.ConvertIntToAlphaID((SELECT COUNT(*) FROM dbo.MyTable))
IF @MaxID IS NULL
RETURN 'AAAAA'
RETURN dbo.ConvertIntToAlphaID(dbo.ConvertAlphaIDToInt(@MaxID)+1)
END
GO
SELECT * FROM dbo.MyTable ORDER BY dbo.ConvertAlphaIDToInt(AlphaID)
GO
SELECT dbo.GetNextAlphaID () AS 'NextAlphaID'
By the way, if you don't want to assume contiguity, you can do as you suggested and (if there's a 'ZZZZZ' row) use the first gap in the sequence. Replace the last function with this:
IF OBJECT_ID('dbo.GetNextAlphaID_2','FN') IS NOT NULL
DROP FUNCTION dbo.GetNextAlphaID_2
GO
CREATE FUNCTION dbo.GetNextAlphaID_2 ()
RETURNS char(5)
AS
BEGIN
DECLARE @MaxID char(5), @ReturnVal char(5)
SELECT @MaxID = MAX(AlphaID) FROM dbo.MyTable
IF @MaxID < 'ZZZZZ'
RETURN dbo.ConvertIntToAlphaID(dbo.ConvertAlphaIDToInt(@MaxID)+1)
IF @MaxID IS NULL
RETURN 'AAAAA'
SELECT TOP 1 @MaxID=M1.AlphaID
FROM dbo.Mytable M1
WHERE NOT EXISTS (SELECT 1 FROM dbo.MyTable M2
WHERE AlphaID = dbo.ConvertIntToAlphaID(dbo.ConvertAlphaIDToInt(M1.AlphaID) + 1 )
)
ORDER BY M1.AlphaID
IF @MaxID IS NULL
RETURN 'AAAAA'
RETURN dbo.ConvertIntToAlphaID(dbo.ConvertAlphaIDToInt(@MaxID)+1)
END
GO
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To return the next ID for a given ID (with rollover), use:
SELECT COALESCE
(
(
SELECT TOP 1 id
FROM mytable
WHERE id > @id
ORDER BY
id
),
(
SELECT TOP 1 id
FROM mytable
ORDER BY
id
)
) AS nextid
This query searches for the ID next to the given. If there is no such ID, it returns the first ID.
Here are the results:
WITH mytable AS
(
SELECT 'AAA' AS id
UNION ALL
SELECT 'BBB' AS id
UNION ALL
SELECT 'CCC' AS id
UNION ALL
SELECT 'DDD' AS id
UNION ALL
SELECT 'EEE' AS id
)
SELECT mo.id,
COALESCE
(
(
SELECT TOP 1 id
FROM mytable mi
WHERE mi.id > mo.id
ORDER BY
id
),
(
SELECT TOP 1 id
FROM mytable mi
ORDER BY
id
)
) AS nextid
FROM mytable mo
id nextid
----- ------
AAA BBB
BBB CCC
CCC DDD
DDD EEE
EEE AAA
, i. e. it returns BBB for AAA, CCC for BBB, etc., and, finally, AAA for EEE which is last in the table.
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I think the lowest-impact solution for my needs is to add an identity column. The one thing I can guarantee is that the ordering will be such that entries that should "come first" will be added first -- I'll never add one with identifier BBBB, then go back and add BBBA later. If I didn't have that constraint, obviously it wouldn't work, but as it stands, I can just order by the identity column and get the sort I want.
I'll keep thinking about the other suggestions -- maybe if they "click" in my head, they'll look like a better option.
