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AUTO Mode Heuristics in Shaping Returned XML

AUTO mode determines the shape of returned XML based on the query. In determining how elements are to be nested, AUTO mode heuristics compare column values in adjacent rows. Columns of all types, except ntext, text, image, and xml, are compared. Columns of type (n)varchar(max) and varbinary(max) are compared.

The following example illustrates the AUTO mode heuristics that determine the shape of the resulting XML:

SELECT T1.Id, T2.Id, T1.Name
FROM   T1, T2
WHERE ...
FOR XML AUTO
ORDER BY T1.Id

To determine where a new <T1> element starts, all column values of T1, except ntext, text, image and xml, are compared if the key on the table T1 is not specified. Next, assume that the Name column is nvarchar(40) and the SELECT statement returns this rowset:

T1.Id  T1.Name  T2.Id
-----------------------
1       Andrew    2
1       Andrew    3
1       Nancy     4

The AUTO mode heuristics compare all the values of table T1, the Id and Name columns. Because the first two rows have the same values for the Id and Name columns, one <T1> element having two <T2> child elements is added in the result.

Following is the XML that is returned:

<T1 Id="1" Name="Andrew">
    <T2 Id="2" />
    <T2 Id="3" />
</T1>
<T1 Id="1" Name="Nancy" >
      <T2 Id="4" />
</T>

Now assume that the Name column is of text type. The AUTO mode heuristics do not compare the values for this type. Instead, it assumes that the values are not the same. This results in XML generation as shown in the following:

<T1 Id="1" Name="Andrew" >
  <T2 Id="2" />
</T1>
<T1 Id="1" Name="Andrew" >
  <T2 Id="3" />
</T1>
<T1 Id="1" Name="Nancy" >
  <T2 Id="4" />
</T1>

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