Question

I have a table called People with a columns of datatype xml called properties. I’ve used this to store random information about each person basically allowing people to store any extra data that are added in the future without a database redesign. Not all people will have the same elements in their xml.

CREATE TABLE [dbo].[Person](
 [PersonID] [bigint] IDENTITY(1,1) NOT NULL,
 [PersonType] [nvarchar](50) NULL,
 [Title] [nvarchar](5) NULL,
 [Forename] [nvarchar](60) NULL,
 [Surname] [nvarchar](60) NULL,
 [Company] [nvarchar](60) NULL,
 [Properties] [xml] NULL
)

An example of the xml is:

<PropertyList xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Property Name="Class">Class A</Property>
  <Property Name="CarRegistration">123456</Property>
  <Property Name="MedicalNotes">None</Property>
</PropertyList>

First question is I can’t seem to find a SQL query that will allow me to get a list of records that match criteria stored in the xml.

For example how would i get all records where the Class="Class A". I’ve tried :

SELECT 
    PersonID, 
    Properties.value('/PropertyList/Property[@Name="Class"][1]','nvarchar(50)') 
FROM Person

I know this is incorrect but I get the error “requires a singleton (or empty sequence)” and I’m not quite sure whats gone wrong.

And second side question is I’ve combined several older databases into a single person list however the old database frontends still need to access their bit of the data. My plan was to create a view for each database frontend with a layout specific for its needs all linking back to the main people table. However some of their fields are now stored in the XML. Is there any way of creating a view to update the XML without seeing the xml i.e. so it looks and acts just like a view on any other table. I hope i explained that correctly. For each view I will have a specific set of XML properties I need them to edit and all records will have them so its not so random.

Thanks for any help.

Answer 1

Your XQuery should be:

SELECT PersonID, 
       Properties.value('(/PropertyList/Property[@Name="Class"])[1]','NVARCHAR(50)') 
FROM dbo.Person

Does that help??

Update: to make it clearer for others – I’ve added parenthesis around the /PropertyList/Property[@Name="Class"] expression, so that this will evaluate to potentially a list of values, and then the [1] after the parenthesis will select the first (and most often only) value (as a singleton) of that list so it can be converted to a NVARCHAR(50) string.

value('(/PropertyList/Property[@Name="Class"])[1]','NVARCHAR(50)') 
       !                                     !

is NOT the same as

value('/PropertyList/Property[@Name="Class"][1]','NVARCHAR(50)') 

Update 2: if you want to create a view – sure, no reason not to! 😉

You could definitely create something like:

CREATE VIEW dbo.YourViewName
AS
    SELECT
        PersonID, PersonType, Title, 
        ForeName, Surname, Company,
        Properties.value('(/PropertyList/Property[@Name="Class"])[1]','NVARCHAR(50)') AS 'Class',
        Properties.value('(/PropertyList/Property[@Name="CarRegistration"])[1]','NVARCHAR(50)') AS 'CarRegistration',
        Properties.value('(/PropertyList/Property[@Name="MedicalNotes"])[1]','NVARCHAR(50)') AS 'MedicalNotes'

from your table and “break up” the XML into columns on your view. Is that what you’re looking for??