SQL Server 2005 ROW_NUMBER() without ORDER BY

Question

I am trying to insert from one table into another using

DECLARE @IDOffset int;
SELECT @IDOffset = MAX(ISNULL(ID,0)) FROM TargetTable

INSERT INTO TargetTable         


        

Answer 1

You can avoid specifying an explicit ordering as follows:

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};

However, please note that is merely a way to avoid specifying an ordering and does NOT guarantee that any original data ordering will be preserved. There are other factors that can cause the result to be ordered, such as an ORDER BY in the outer query. To fully understand this, one must realize that the concept "not ordered (in a particular way)" is not the same as "retaining original order" (which IS ordered in a particular way!). I believe that from a pure relational database perspective, the latter concept does not exist, by definition (though there may be database implementations that violate this, SQL Server is not one of them).

The reason for the lock hints is to prevent the case where some other process inserts using the value you plan to use, in between the parts of the query executing.

Note: Many people use (SELECT NULL) to get around the "no constants allowed in the ORDER BY clause of a windowing function" restriction. For some reason, I prefer 1 over NULL.

Also: I think an identity column is far superior and should be used instead. It's not good for concurrency to exclusively lock entire tables. Understatement.

Answer 2

You can ignore the ordering by using order by (select null) like this:

declare @IDOffset int;
select  @IDOffset = max(isnull(ID, 0)) from TargetTable

insert  into TargetTable(ID, FIELD)
select  row_number() over (order by (select null)) + @IDOffset, FeildValue
  from  SourceTable
 where  [somecondition]