Question

I have a table with an Xml column in Sql. All Xml files have a same schema and I want to merge some of this Xml together.

For example for X1:

 <A>
     <B>
         <C id='101'>
             <D id='102'>abcd</D>
         </C>
         <C id='103'>
             <D id='104'>zxcv</D>
         </C>
     </B>
 </A>

and X2:

 <A>
     <B>
         <C id='101'>
             <D id='102'>abcd</D>
             <D id='501'>abef</D>
         </C>
         <C id='502'>
             <D id='503'>efgh</D>
         </C>
     </B>
 </A>

X1+X2=…

 <A>
     <B>
         <C id='101'>
             <D id='102'>abcd</D>
             <D id='501'>abef</D>
         </C>
         <C id='103'>
             <D id='104'>zxcv</D>
         </C>            
         <C id='502'>
             <D id='503'>efgh</D>
         </C>
     </B>
 </A>

So which choice is the best and how:

• XQuery in Sql
• C# XDocument and XPath
• …

Answer 1

I think the best way to approach this is to write a class that merges two XDocuments using the visitor pattern, with the distinction that we are always visiting nodes from the first document in parallel with nodes from the second document.

The overall design would be something like:

class XmlMerger
{
    public XDocument Merge(XDocument first, XDocument second);

    private XElement MergeElements(XElement first, XElement second);

    private XAttribute MergeAttributes(XAttribute first, XAttribute second);

    private XText MergeTexts(XText first, XText second);
}

A specific implementation could look like this:

class XmlMerger
{
    public XDocument Merge(XDocument first, XDocument second)
    {
        return new XDocument(MergeElements(first.Root, second.Root));
    }

    private XElement MergeElements(XElement first, XElement second)
    {
        if (first == null)
            return second;

        if (second == null)
            return first;

        if (first.Name != second.Name)
            throw new InvalidOperationException();

        var firstId = (string)first.Attribute("id");
        var secondId = (string)second.Attribute("id");

        // different ids
        if (firstId != secondId)
            throw new InvalidOperationException();

        var result = new XElement(first.Name);

        var attributeNames = first.Attributes()
            .Concat(second.Attributes())
            .Select(a => a.Name)
            .Distinct();

        foreach (var attributeName in attributeNames)
            result.Add(
                MergeAttributes(
                    first.Attribute(attributeName),
                    second.Attribute(attributeName)));

        // text-only elements
        if (first.Nodes().OfType<XText>().Any() ||
            second.Nodes().OfType<XText>().Any())
        {
            var firstText = first.Nodes().OfType<XText>().FirstOrDefault();
            var secondText = second.Nodes().OfType<XText>().FirstOrDefault();

            // we're not handling mixed elements
            if (first.Nodes().Any(n => n != firstText) ||
                second.Nodes().Any(n => n != secondText))
                throw new InvalidOperationException();

            result.Add(MergeTexts(firstText, secondText));
        }
        else
        {
            var elementNames = first.Elements()
                .Concat(second.Elements())
                .Select(e => e.Name)
                .Distinct();

            foreach (var elementName in elementNames)
            {
                var ids = first.Elements(elementName)
                    .Concat(second.Elements(elementName))
                    .Select(e => (string)e.Attribute("id"))
                    .Distinct();

                foreach (var id in ids)
                {
                    XElement firstElement = first.Elements(elementName)
                        .SingleOrDefault(e => (string)e.Attribute("id") == id);
                    XElement secondElement = second.Elements(elementName)
                        .SingleOrDefault(e => (string)e.Attribute("id") == id);

                    result.Add(MergeElements(firstElement, secondElement));
                }
            }
        }

        return result;
    }

    private XAttribute MergeAttributes(XAttribute first, XAttribute second)
    {
        if (first == null)
            return second;

        if (second == null)
            return first;

        if (first.Name != second.Name)
            throw new InvalidOperationException();

        if (first.Value == second.Value)
            return new XAttribute(first);

        // can't merge attributes with different values
        throw new InvalidOperationException();
    }

    private XText MergeTexts(XText first, XText second)
    {
        if (first == null)
            return second;

        if (second == null)
            return first;

        if (first.Value == second.Value)
            return new XText(first);

        // can't merge texts with different values
        throw new InvalidOperationException();
    }
}

If this code encounters something it can’t handle (e.g. nodes with the same id but different text; or comments), it throws an exception.