SQL Server: Existing column and value incrementing - sql

Question

I'm trying to create and increment by one some values to put into an already existing (but empty) column. I'm currently using the identity function, but I wouldn't mind using a custom made function. Right now, SSMS is saying there's incorrect syntax near IDENTITY. Could anybody help me fix this syntax?
ALTER Table anthemID IDENTITY(1,1)

Answer 1

First, you can't make a column identity after the fact: it has to be set that way at creation time.
Second, I'm not quite sure what you mean by "increment the value of an already existing column by one." You can only increment the value of rows within a column--perform a DML (Data Modification Language) query. The script you suggested above is a DDL (Data Definition Language) query that actually modifies the structure of the table, affecting the entire column--all rows.
If you just want to increment all the rows by 1, you'd do this:
UPDATE dbo.YourTable SET anthemID = anthemID + 1;
On the other hand, if you want the anthemID column to acquire the identity property so that new inserts to the table receive unique, autoincrementing values, you can do that with some juggling:
Back up your database and confirm it is a good backup.
Script out your table including all constraints.
Drop all constraints on your table or other tables that involve anthemID.
ALTER TABLE dbo.YourTable DROP CONSTRAINT PK_YourTable -- if part of PK
ALTER TABLE dbo.AnotherTable DROP CONSTRAINT FK_AnotherTable_anthemID -- FKs
Rename your table
EXEC sp_rename 'dbo.YourTable', 'YourTableTemp';
Modify the script you generated above to make anthemID identity (add in identity(1,1) after int);
Run the modified script to create a new table with the same name as the original.
Insert the data from the old table to the new one:
SET IDENTITY_INSERT dbo.YourTable ON;
INSERT dbo.YourTable (anthemID, AnotherColumn, RestOfColumns)
SELECT anthemID, AnotherColumn, RestOfColumns
FROM dbo.YourTableTemp;
SET IDENTITY_INSERT dbo.YourTable OFF;
Re-add all constraints that were dropped.
Drop the original, renamed table after confirming you don't need the data any more.
You may be able to do this from SSMS's GUI table designer, and it will take care of moving the data over for you. However, this has bitten some people in the past and if you don't have a good database backup, well, don't do it because you might encounter some regret in the process.
UPDATE
Now that I know the column is blank, it's even easier.
ALTER TABLE dbo.YourTable DROP COLUMN anthemID;
ALTER TABLE dbo.YourTable ADD anthemID int identity(1,1) NOT NULL;
This does have the drawback of moving the column to the end of the table. If that's a problem, you can follow much the same procedure as I outlined above (to fix things yourself, or alternately use the designer in SQL Server Management Studio).
I recommend in the strongest terms possible that you use an identity column and do not try to create your own means of making new rows get an incremented value.
For emphasis, I'll quote #marc_s's comment above:
The SELECT MAX(ID)+1 approach is highly unsafe in a concurrent environment - in a system under some load, you will get duplicates. Don't do this yourself - don't try to reinvent the wheel - use the proper mechanisms (here: IDENTITY) that your database gives you and let the database handle all the nitty-gritty details!
I wholeheartedly agree with him.