Querying Serialized Dictionary in XML Column

Question

I have a Dictionary that I'm serializing in C# and storing in an XML column on MSSQL Server. The serialized XML looks like this:

<ArrayOfKeyValueOfstringanyType xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
  <KeyValueOfstringanyType>
     <Key>code</Key><Value xmlns:d3p1="http://www.w3.org/2001/XMLSchema" i:type="d3p1:string">WFR 052</Value>
  </KeyValueOfstringanyType>
  <KeyValueOfstringanyType>
    <Key>type</Key><Value xmlns:d3p1="http://www.w3.org/2001/XMLSchema" i:type="d3p1:string">Newsletter</Value>
  </KeyValueOfstringanyType>
</ArrayOfKeyValueOfstringanyType>

Ultimately, I want to get the Value, where the Key is "code". The first step I took was to just get the first value, regardless of key.

SELECT [xml_column].value('(/ArrayOfKeyValueOfstringanyType/KeyValueOfstringanyType/Value)[1]','varchar(255)') as val
FROM [my_table]

I get null values back. I know it has something to do with the namespaces, because when I try the same query with the namespaces removed, I get a value. I've seen some other scenarios with namespaces, but my XML format is a bit different, and I'm struggling to find the right syntax.

Here's another question I looked at:

XML Field - Query

Answer 1

The problem you are having is due to the schema specification in your XML. If a node in your XML document is part of a schema you have to specify that schema when querying that node. Alternatively, you can use a wildcard for the schema specification. But specifying the node name without schema does not work (as you have experienced).

So let's look at an example:

SQL Fiddle

MS SQL Server 2008 Schema Setup:

CREATE TABLE dbo.Tbl(id INT IDENTITY(1,1) PRIMARY KEY CLUSTERED, dict XML);

INSERT INTO dbo.Tbl(dict)
VALUES('<ArrayOfKeyValueOfstringanyType xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
      <KeyValueOfstringanyType>
         <Key>code</Key><Value xmlns:d3p1="http://www.w3.org/2001/XMLSchema" i:type="d3p1:string">WFR 052</Value>
      </KeyValueOfstringanyType>
      <KeyValueOfstringanyType>
        <Key>type</Key><Value xmlns:d3p1="http://www.w3.org/2001/XMLSchema" i:type="d3p1:string">Newsletter</Value>
      </KeyValueOfstringanyType>
    </ArrayOfKeyValueOfstringanyType>');

The table dbo.Tbl is created with just two columns, an identity id column and a dict column for the XML.

To get your first query to work, specify the schema using a wildcard for each node:

Query 1:

SELECT dict.value('/*:ArrayOfKeyValueOfstringanyType[1]/*:KeyValueOfstringanyType[1]/*:Key[1]','NVARCHAR(MAX)')
FROM dbo.Tbl;

That results in the first Key being returned:

Results:

| COLUMN_0 |
|----------|
|     code |

Now, you want to return the Value nodes for all key value pairs where the Key = 'code'. You could do that filtering in xquery, but I usually prefer filtering in SQL. For that we first need to get all pairs back. The XML nodes function gets us a step closer:

Query 2:

SELECT id,key_value.query('.')
  FROM dbo.Tbl
 CROSS APPLY dict.nodes('/*:ArrayOfKeyValueOfstringanyType/*:KeyValueOfstringanyType') AS N(key_value);

It returns one row per KeyValueOfstringanyType node:

Results:

| ID |                                                                                                                                                                                                                                                                                                      COLUMN_1 |
|----|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
|  1 |    <p1:KeyValueOfstringanyType xmlns:p1="http://schemas.microsoft.com/2003/10/Serialization/Arrays"><p1:Key>code</p1:Key><p1:Value xmlns:d3p1="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="d3p1:string">WFR 052</p1:Value></p1:KeyValueOfstringanyType> |
|  1 | <p1:KeyValueOfstringanyType xmlns:p1="http://schemas.microsoft.com/2003/10/Serialization/Arrays"><p1:Key>type</p1:Key><p1:Value xmlns:d3p1="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="d3p1:string">Newsletter</p1:Value></p1:KeyValueOfstringanyType> |

Using that, we can get to the Key and Value using the XML.value function: Query 3:

SELECT id,
       key_value.value('./*:Key[1]','NVARCHAR(MAX)') AS [key],
       key_value.value('./*:Value[1]','NVARCHAR(MAX)') AS [value]
  FROM dbo.Tbl
 CROSS APPLY dict.nodes('/*:ArrayOfKeyValueOfstringanyType/*:KeyValueOfstringanyType') AS N(key_value);

Now we have a row per key value pair with the key and the value in separate columns:

Results:

| ID |  KEY |      VALUE |
|----|------|------------|
|  1 | code |    WFR 052 |
|  1 | type | Newsletter |

From there is is easy to apply additional filters in a WHERE clause:

Query 4:

WITH KeyValues AS(
SELECT id,
       key_value.value('./*:Key[1]','NVARCHAR(MAX)') AS [key],
       key_value.value('./*:Value[1]','NVARCHAR(MAX)') AS [value]
  FROM dbo.Tbl
 CROSS APPLY dict.nodes('/*:ArrayOfKeyValueOfstringanyType/*:KeyValueOfstringanyType') AS N(key_value)
)
SELECT *
  FROM KeyValues
 WHERE [Key] = 'code';

Results:

| ID |  KEY |   VALUE |
|----|------|---------|
|  1 | code | WFR 052 |