如何从NVARCHAR（MAX）属性解析编码为UTF-8的XML？

Question

I'm facing a problem to parse an XML string stored in a field of type NVARCHAR(MAX) (I cannot change the type of this field).

Here is my table (WorkingHours) :

CREATE TABLE WorkingHours(
    [ID] [int] NOT NULL PRIMARY KEY,
    [CONTENT] [nvarchar](MAX) NOT NULL,
    -- ...
);

Here is a sample of the [CONTENT] attribute :

<?xml version="1.0" encoding="UTF-8"?>
    <calendar>
        <day number="1" worked_day="no">
            <interval number="1" begin_hour="08:30" end_hour="12:00"/>
            <interval number="2" begin_hour="13:30" end_hour="17:00"/>
            <interval number="3" begin_hour="" end_hour=""/></day>
        <day number="2" worked_day="no">
            <interval number="1" begin_hour="08:30" end_hour="12:00"/>
            <interval number="2" begin_hour="13:30" end_hour="17:00"/>
            <interval number="3" begin_hour="" end_hour=""/>
        </day>
        <day number="3" worked_day="no">
            <interval number="1" begin_hour="08:30" end_hour="12:00"/>
            <interval number="2" begin_hour="13:30" end_hour="17:00"/>
            <interval number="3" begin_hour="" end_hour=""/>
        </day>
        <day number="4" worked_day="no">
            <interval number="1" begin_hour="08:30" end_hour="12:00"/>
            <interval number="2" begin_hour="13:30" end_hour="17:00"/>
            <interval number="3" begin_hour="" end_hour=""/>
        </day>
        <day number="5" worked_day="no">
            <interval number="1" begin_hour="08:30" end_hour="12:00"/>
            <interval number="2" begin_hour="13:30" end_hour="17:00"/>
            <interval number="3" begin_hour="" end_hour=""/>
        </day>
        <day number="6" worked_day="no">
            <interval number="1" begin_hour="" end_hour=""/>
            <interval number="2" begin_hour="" end_hour=""/>
            <interval number="3" begin_hour="" end_hour=""/>
        </day>
        <day number="7" worked_day="no">
            <interval number="1" begin_hour="" end_hour=""/>
            <interval number="2" begin_hour="" end_hour=""/>
            <interval number="3" begin_hour="" end_hour=""/>
        </day>
    </calendar>

As you can see, the data encoding is UTF-8 .

Now, I would like to parse this data in order to create some calculations :

DECLARE @RawContent [nvarchar](MAX) = (
    SELECT wh.[CONTENT]
    FROM [WorkingHours] wh 
    WHERE wh.[ID] = 100);

DECLARE @XMLContent [Xml] = @RawContent; // KO
-- DECLARE @XMLContent [Xml] = CAST(@RawContent AS XML);  // KO
-- DECLARE @XMLContent [Xml] = CONVERT(XML, @RawContent); // KO

-- Just a test to query XML data.
SELECT 
    C.WD.value('@number', 'int') AS DayId         
FROM @XMLContent.nodes('/calendar/day') AS C(WD);   

I don't know how to cast the result (a nvarchar(max) field containing UTF-8 XML string) to a XML value. SQL Server returns the following error :

"Unable to switch encoding"

It refers to the CAST line (when I define the @XMLContent variable).

Any idea to solve that ?

Answer 1

Strip out the processing directive -- it's meaningless and incorrect because the data is already encoded in UTF-16 (since it's stored as NVARCHAR ). If you cannot change the data already present, you'll have to rely on (slightly brittle) string replacement:

CAST(REPLACE(wh.[CONTENT], '<?xml version="1.0" encoding="UTF-8"?>', '') AS XML)

Note that explicitly indicating the encoding is UTF-16 instead will also work -- though it adds nothing.

Answer 2

The other option is to convert to a VARCHAR datatype first - which is non-Unicode - and then to XML :

DECLARE @RawContent [nvarchar](MAX) = (
    SELECT wh.[CONTENT]
    FROM [WorkingHours] wh 
    WHERE wh.[ID] = 100);

DECLARE @XMLContent XML = CAST(CAST(@RawContent AS VARCHAR(MAX)) AS XML)

-- Just a test to query XML data.
SELECT 
    C.WD.value('@number', 'int') AS DayId         
FROM @XMLContent.nodes('/calendar/day') AS C(WD);