How Can I Auto-Increment Data Without an Identity Column Using ROW_NUMBER()?

Auto-Incrementing Data Using ROW_NUMBER() without ORDER BY

When inserting data into a table without an identity column, it becomes necessary to find alternative methods for auto-incrementing the primary key. One commonly used approach is the ROW_NUMBER() function, which allows for sequential numbering of rows.

Traditionally, the ROW_NUMBER() function requires an ORDER BY clause to determine the order of rows. However, there are scenarios where a legitimate ordering field is not available. To address this, a workaround exists that involves using an arbitrary constant expression in the ORDER BY clause, such as (SELECT 1).

By combining this technique with the OVER() clause, it becomes possible to calculate the row number for each row without specifying a specific order:

Row_Number() OVER (ORDER BY (SELECT 1))

This expression can then be used in an INSERT statement to auto-increment the primary key:

INSERT INTO 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};

However, it is important to note that this workaround does not guarantee the preservation of the original ordering of rows from the SourceTable. In fact, there may be other factors that influence the final ordering of the result. This is because the concept of "not ordered (in a particular way)" differs from "retaining original order".

Therefore, using an identity column for primary key auto-incrementing is generally considered superior to the ROW_NUMBER() workaround. It not only eliminates the need for complex calculations but also improves concurrency and data integrity by ensuring that each row has a unique and non-conflicting ID.

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