How Can I Auto-Increment a Non-Identity Column in SQL Server 2005 Without Using ORDER BY in ROW_NUMBER()?

Avoiding ORDER BY in SQL Server 2005 ROW_NUMBER()

When inserting data from one table into another, it's often desirable to auto-increment a non-identity column in the target table. While the ROW_NUMBER() function can be used to achieve this, it typically requires an ORDER BY field. However, if the original order of the source table must be preserved, it's necessary to find an alternative approach.

A solution that avoids explicit ordering involves the following code:

INSERT dbo.TargetTable (ID, FIELD)
SELECT
   Row_Number() OVER (ORDER BY (SELECT 1))
      + Coalesce(
         (SELECT Max(ID) FROM dbo.TargetTable WITH (TABLOCKX, HOLDLOCK)),
         0
      ),
   FieldValue
FROM dbo.SourceTable
WHERE {somecondition};

This code utilizes a window function to calculate row numbers based on a constant value, effectively replacing the need for an ORDER BY clause. Additionally, to prevent concurrency issues, the query employs lock hints (TABLOCKX and HOLDLOCK) while calculating the maximum value in the target table.

It's important to note that while this approach avoids explicitly specifying an ordering, it doesn't guarantee that the original order of the source table will be preserved. Factors such as outer query orderings can still influence the result. From a database perspective, the concept of "retaining original order" doesn't truly exist.

For optimal concurrency and data integrity, the use of an identity column is highly recommended over this workaround. Exclusive table locking through the lock hints should be avoided unless absolutely necessary.

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