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Converting Relational Tables to XML documents using FOR XML EXPLICIT

Continuing with the series on XML, today we will talk about the FOR XML EXPLICIT clause which can be used with relation database queries to produce XML output. For a complete list of related topics, please refer here.

As mentioned above, we can use the FOR XML EXPICIT clause in the T-SQL queries to produce XML output. FOR XML has different flavors which can be used to produced desired results. Examples for SQL 2000 use the pubs/Northwind database, while SQL 2005 and the SQL 2008 examples are created over the AdventureWorks database which can be downloaded online.

Using the RAW and AUTO mode, we did not have any control over the Nesting of Elements in the XML result. The EXPLICIT mode, provides us more control over the nesting of the elements, but doing so requires a careful understanding of what is the desired nesting level and how to implement the same in your Query.

To use the EXPLICIT mode, we need to write the query in a way to produce the universal table. This table is nothing but a relational structure defining the Nesting of the Elements. Proper order by clauses have to be used to make sure that a Parent and all its children and their children are all together in the universal table.

To accomplish this, we have to always define the 1st and 2nd column in our select query as the TAG and PARENT column. The TAG column will indicate the Nest Level for each element in the XML, while the Parent column would be used to represent the Parent/Child relationship.

An example of a universal table is presented below.

Tag Parent Employee!1!EmpID EmpName!2!FName EmpName!2!LName
1 NULL 1 NULL NULL
2 1 1 Guy Gilbert
1 NULL 2 NULL NULL
2 1 2 Kevin Brown
1 NULL 3 NULL NULL
2 1 3 Roberto Tamburello
1 NULL 4 NULL NULL
2 1 4 Rob Walters
1 NULL 5 NULL NULL
2 1 5 Thierry D’Hers

When SQL Server tries to convert this output into XML, it will pick all ELEMENTS with TAG value of 1 and Parent value of 0, and make them Root Elements. Similarly, elements with TAG value 2 and Parent Value 1, and occurring immediately after the Root Element will be made a child of the Root Element, and so on.

Once again the order of data in the universal table will define the nesting. For Example consider the query below, where I have ordered the result using TAG column.

SELECT 1                    as Tag,
         NULL                 as Parent,
         Customers.CustomerID as [Customer!1!CustomerID],
         NULL                 as [Order!2!OrderID]
FROM Customers
UNION ALL
SELECT 2,
         1,
         Customers.CustomerID,
         Orders.OrderID
FROM Customers, Orders
WHERE Customers.CustomerID = Orders.CustomerID
ORDER BY TAG

<<<<<<<<<<<<<<<<<<<<<< Universal Table Snippet >>>>>>>>>>>>>>>

TAG Parent

Customer!1!CustomerID

Order!2!OrderID

1 NULL WANDK NULL
1 NULL WARTH NULL
1 NULL WELLI NULL
1 NULL WHITC NULL
1 NULL WILMK NULL
1 NULL WOLZA NULL
2 1 ALFKI 10643
2 1 ALFKI 10692
2 1 ALFKI 10702
2 1 ALFKI 10835
2 1 ALFKI 10952

In this case, SQL will create an XML with multiple root nodes, with the Last root Node having multiple child!

Also note the naming mechanism of the columns used with EXPLICIT mode. The Columns have to named as

ElementName!TagNumber!AttributeName!directive

As with RAW and AUTO mode, EXPLICIT mode can also be used in conjunction some directives. SQL 2000 directives are listed below.

ID : An element attribute can be specified to be an ID type attribute. IDREF and IDREFS attributes can then be used to refer to them, enabling intra-document links. If XMLDATA is not requested, this keyword has no effect.

IDREF : Attributes specified as IDREF can be used to refer to ID type attributes, enabling intra-document links. If XMLDATA is not requested, this keyword has no effect.

IDREFS: Attributes specified as IDREFS can be used to refer to ID type attributes, enabling intra-document links. If XMLDATA is not requested, this keyword has no effect.

HIDE: The attribute is not displayed. This may be useful for ordering the result by an attribute that will not appear in the result.

ELEMENT: This does not generate an attribute. Instead it generates a contained element with the specified name (or generate contained element directly if no attribute name is specified). The contained data is encoded as an entity (for example, the < character becomes &lt;). This keyword can be combined with ID, IDREF, or IDREFS.

XML: This is the same as an element directive except that no entity encoding takes place (for example, the < character remains <). This directive is not allowed with any other directive except hide.

XMLTEXT: The column content should be wrapped in a single tag that will be integrated with the rest of the document. This directive is useful in fetching overflow (unconsumed) XML data stored in a column by OPENXML. If AttributeName is specified, the tag name is replaced by the specified name; otherwise, the attribute is appended to the current list of attributes of the enclosing elements and by putting the content at the beginning of the containment without entity encoding. The column with this directive must be a text type (varchar, nvarchar, char, nchar, text, ntext). This directive can be used only with hide. This directive is useful in fetching overflow data stored in a column. If the content is not a well-formed XML, the behavior is undefined.

CDATA: Contains the data by wrapping it with a CDATA section. The content is not entity encoded. The original data type must be a text type (varchar, nvarchar, text, ntext). This directive can be used only with hide. When this directive is used, AttributeName must not be specified.

I will be using some of these directives in a single query to display the results.

SELECT 1                      as Tag,
         NULL                 as Parent,
         Customers.CustomerID as [Customer!1!CustomerID!ID],
     NULL               as [CustomerName!2!Name!IDREF],
     NULL                  as [CustomerName!2!Phone!Hide],
         NULL                 as [Order!3!OrderID],
     NULL              as [Order!3!OrderDate!ELEMENT]
FROM Customers
UNION ALL
SELECT 2,
         1,
         Customers.CustomerID,
     Customers.ContactName,
     Customers.Phone,   
         NULL,
     NULL
FROM Customers
UNION ALL
SELECT 3,
    2,
    Customers.CustomerID,
    Customers.ContactName,
    NULL,
    Orders.OrderID,
    Orders.OrderDate
FROM Customers, Orders
WHERE Customers.CustomerID = Orders.CustomerID
ORDER BY [Customer!1!CustomerID!ID],[CustomerName!2!Name!IDREF], [Order!3!OrderID]
FOR XML EXPLICIT, XMLDATA

<<<<<<<<<<<<<<<<<< Snippet Output >>>>>>>>>>>>>>>>>

<Schema name="Schema5" xmlns="urn:schemas-microsoft-com:xml-data" xmlns:dt="urn:schemas-microsoft-com:datatypes">
<ElementType name="Customer" content="mixed" model="open">
        <AttributeType name="CustomerID" dt:type="id"/>
        <attribute type="CustomerID"/>
</ElementType>
<ElementType name="CustomerName" content="mixed" model="open">
    <AttributeType name="Name" dt:type="idref"/><attribute type="Name"/>
</ElementType>
<ElementType name="Order" content="mixed" model="open">
    <AttributeType name="OrderID" dt:type="i4"/><attribute type="OrderID"/>
<Element type="OrderDate"/></ElementType>
    <ElementType name="OrderDate" content="textOnly" model="closed" dt:type="dateTime"/>
    </ElementType>
</ElementType>
</Schema>
<Customer xmlns="x-schema:#Schema5" CustomerID="ALFKI">
    <CustomerName Name="Maria Anders">
        <Order OrderID="10643">
            <OrderDate>1997-08-25T00:00:00</OrderDate>
        </Order>
        <Order OrderID="10692">
            <OrderDate>1997-10-03T00:00:00</OrderDate>
        </Order>
        <Order OrderID="10702">
            <OrderDate>1997-10-13T00:00:00</OrderDate>
        </Order>
        <Order OrderID="10835">
            <OrderDate>1998-01-15T00:00:00</OrderDate>
        </Order>

<<<<<<<<<<<<<<<<<< Snippet Output>>>>>>>>>>>>>>>>>>>>

The same directives and constructs can be used with SQL 2005 and SQL 2008.

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One response to “Converting Relational Tables to XML documents using FOR XML EXPLICIT

  1. Pingback: Working with XML in Microsoft SQL Server « SQLUninterrupted

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