sql server - Using SQL to transpose/flatten XML structure to columns

Question

I am using SQL Server (2008/2012) and I know there are similar answers from lots of searching, however I can't seem to find the appropriate example/pointers for my case.

I have an XML column in a SQL Server table holding this data:

<Items>
 <Item>
  <FormItem>
    <Text>FirstName</Text>
    <Value>My First Name</Value>
  </FormItem>
  <FormItem>
    <Text>LastName</Text>
    <Value>My Last Name</Value>
  </FormItem>
  <FormItem>
    <Text>Age</Text>
    <Value>39</Value>
  </FormItem>
 </Item>
 <Item>
  <FormItem>
    <Text>FirstName</Text>
    <Value>My First Name 2</Value>
  </FormItem>
  <FormItem>
    <Text>LastName</Text>
    <Value>My Last Name 2</Value>
  </FormItem>
  <FormItem>
    <Text>Age</Text>
    <Value>40</Value>
  </FormItem>
 </Item>
</Items>

So even though the structure of <FormItem> is going to be the same, I can have multiple (most commonly no more than 20-30) sets of form items..

I am essentially trying to return a query from SQL in the format below, i.e. dynamic columns based on /FormItem/Text:

FirstName         LastName         Age    ---> More columns as new `<FormItem>` are returned
My First Name     My Last Name     39          Whatever value etc..
My First Name 2   My Last Name 2   40          

So, at the moment I had the following:

select 
    Tab.Col.value('Text[1]','nvarchar(100)') as Question,
    Tab.Col.value('Value[1]','nvarchar(100)') as Answer
from
    @Questions.nodes('/Items/Item/FormItem') Tab(Col)

Of course that hasn't transposed my XML rows into columns, and obviously is fixed with fields anyway.. I have been trying various "Dynamic SQL" approaches where the SQL performs a distinct selection of (in my case) the <Text> node, and then uses some sort of Pivot? but I couldn't seem to find the magic combination to return the results I need as a dynamic set of columns for each row (<Item> within the collection of <Items>).

I'm sure it can be done having seen so many very similar examples, however again the solution eludes me!

Any help gratefully received!!

Answer 1

Parsing the XML is fairly expensive so instead of parsing once to build a dynamic query and once to get the data you can create a temporary table with a Name-Value list and then use that as the source for a dynamic pivot query.
dense_rank is there to create the ID to pivot around.
To build the column list in the dynamic query it uses the for xml path('') trick.

This solution requires that your table has a primary key (ID). If you have the XML in a variable it can be somewhat simplified.

select dense_rank() over(order by ID, I.N) as ID,
       F.N.value('(Text/text())[1]', 'varchar(max)') as Name,
       F.N.value('(Value/text())[1]', 'varchar(max)') as Value
into #T
from YourTable as T
  cross apply T.XMLCol.nodes('/Items/Item') as I(N)
  cross apply I.N.nodes('FormItem') as F(N)

declare @SQL nvarchar(max)
declare @Col nvarchar(max)

select @Col = 
  (
  select distinct ','+quotename(Name)
  from #T
  for xml path(''), type
  ).value('substring(text()[1], 2)', 'nvarchar(max)')

set @SQL = 'select '+@Col+'
            from #T
            pivot (max(Value) for Name in ('+@Col+')) as P'

exec (@SQL)

drop table #T

SQL Fiddle