trying to turn XML into heirarchical SQL tables, can anyone provide a good explanation of two parts of this code example?

Question

I am trying to learn how to turn XML into heirarchical SQL tables.

I have found an old code snippet from a Microsoft forum which is basically what I am trying to do, but I was wondering if someone could help figure out what is going on line by line on this code, particularly after the xml has been loaded onto @XML

---

--I understand this part, just making the tables  

DECLARE @Books TABLE (BookID int identity(1,1),BookTitle varchar(50),BookLanguage varchar(20),BookPrice decimal(18,2))  
DECLARE @Topics TABLE (TopicID int identity(1,1),BookID int,TopicTitile varchar(50),Page int)  

--I understand this part, defining the @xml variable to be the xml below.. just a usual xml...


DECLARE @xml XML  
SET @xml = '  
<bookstore>  
    <name>My Bookstore</name><br/>  
    <location>New York</location><br/>  
    <book>  
        <title lang=&quot;eng&quot;>Harry Potter</title>  
        <price>29.99</price>  
        <tableOfContents>  
            <topic>  
                <title>Harry Potter Topic 1</title>  
                <page>2</page>  
            </topic>  
            <topic>  
                <title>Harry Potter Topic 2</title>
                <page>5</page>
            </topic>
        </tableOfContents>
    </book>
    <book>
        <title lang=&quot;eng&quot;>Learning XML</title>
        <price>39.95</price>
        <tableOfContents>
            <topic>
                <title>Learning XML Topic 1</title>
                <page>1</page>
            </topic>
            <topic>
                <title>Learning XML Topic 2</title>
                <page>2</page>
            </topic>
        </tableOfContents>
    </book>
</bookstore>'



--what is going on below here?  I am familiar with inserting data into tables,  
--but what kind of insert is this where you are selecting some things and then doing a  
--from @xml.nodes also, what is that T(c) at the end?  and do we always have to put    
--a [1] after each xml element to denote we are referring to the first one we encounter?  


INSERT INTO @Books  
SELECT T.c.value('title[1]','varchar(50)') AS 'BookTitle',  
    T.c.value('(title/@lang)[1]','varchar(20)') AS 'BookLanguage',  
    T.c.value('price[1]','decimal(18,2)') AS 'BookPrice'  
FROM @xml.nodes('/bookstore/book') T(c)  


--what is going on here as well?  what is n(x) ? 
--could you explain this line by line-ish as well? I ran this on  
--SQL Server Management Studio and noticed that both of the 'topic titles' for each  
--book got inserted.  Where in the code did those get put into the table?

INSERT INTO @Topics  
SELECT b.BookID,n.x.value('title[1]','varchar(50)') AS 'TopicTitile',  
    n.x.value('page[1]','int') AS 'TopicPage'  
FROM @Books b   
cross apply @xml.nodes('/bookstore/book/tableOfContents/topic[../../title=sql:column("b.BookTitle")]') n(x)  


--below here is just regular sql selects so this makes sense.  

SELECT BookID,BookTitle,BookLanguage,BookPrice FROM @Books  
SELECT TopicID,BookID,TopicTitile,Page FROM @Topics  

---

Thanks a lot!

the forum I was referring to and trying to learn from with the old post is:

http://social.msdn.microsoft.com/Forums/en/sqlxml/thread/7216ccc9-c1d7-418d-95a2-ec3a96de2c27

Answer 1

BOL says:

syntax:

nodes (XQuery) as Table(Column)

Here is simple example:

DECLARE @x xml ;
SET @x='<Root>
    <row id="1"><name>Larry</name><oflw>some text</oflw></row>
    <row id="2"><name>moe</name></row>
    <row id="3" />
</Root>';

SELECT T.c.query('.') AS result
FROM   @x.nodes('/Root/row') T(c);
GO

It is a special notician to "convert" or shred xml data type into relational data. It just maps xml parts into the table columns. T - table, c - column, nodes() - method

value (XQuery, SQLType)

So T.c.value('title[1]','varchar(50)') reads value of element title and converts it to varchar(50) data type. [1] is added at the end of the path expression in the value() method to explicitly indicate that the path expression returns a singleton (just it confuses me, it means the first element in group in XPath).

So T.c.value('(title/@lang)[1]','varchar(20)') reads value of attribute lang at element title and converts it into varchar(20) data type.

And @xml.nodes('/bookstore/book') is located the point to start reading xml, in this case it returns all book elements (nodes) from this xml.

This query has 2 aliases T1(Locations) and T2(steps)

ADDED

SELECT 
ProductModelID
, Locations.value('./@LocationID','int') as LocID
, steps.query('.') as Step       
FROM Production.ProductModel       

CROSS APPLY Instructions.nodes('       
declare namespace MI="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelManuInstructions";       
/MI:root/MI:Location') 
as T1(Locations) 

CROSS APPLY T1.Locations.nodes('       
declare namespace MI="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelManuInstructions";       
./MI:step ') 
as T2(steps)       
GO