Sql Server: how to add new identity column and populate column with ids?

Maintaining data integrity

Once the identity column is in place, make it the primary key. This not only ensures data integrity but also improves query performance by invoking uniqueness and forming an implicit clustered index.

ALTER TABLE YourTable ADD CONSTRAINT PK_YourTable PRIMARY KEY CLUSTERED (NewIdColumn);
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Turning NewIdColumn into a primary key means there is no room for NULLs and duplicate values--as it should be!

Dealing with large tables: The sweat-free approach

ALTER TABLE can cause a performance hangover when dealing with large tables. To avoid this, combine the ROW_NUMBER() function with table recreation. Here's the recipe:

/* Put your safety helmet on */
BEGIN TRANSACTION;

/* Clone the old table, but with an extra identity column */
CREATE TABLE TempTable AS
SELECT *, ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) AS NewIdColumn
FROM YourTable;

/* Kick the old table out */
DROP TABLE YourTable;

/* Rename the cloned table as the original one */
EXEC sp_rename 'TempTable', 'YourTable';

/* Construction completed */
COMMIT TRANSACTION;
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This might look like a lot of hustling; but trust me, it's sweat-free! And the best part? Say goodbye to those pesky cursors. Remember, keep your data sequence in check by providing a logical ordering when using ROW_NUMBER().

Consider these before you add an identity column

Adding an identity column could be a game-changer unless she doesn't play well with:

• Replicated tables: Identity values can kick up a storm, leading to replication conflicts;
• Distributed databases: Unique identifiers are a hard nut to crack across systems;
• ETL processes: Watch out! Your transformations may fall apart if they're dependent on a specific schema.