Question

Is anyone able to explain why the two results are different, even though the node XML representation is exactly the same?

declare @t varchar(max) set @t = '<a><b><c/></b><d>eeee</d></a>'
declare @x xml set @x = cast(@t as xml)
select N.query('.'), N.value('local-name(.)','varchar(max)')+'.' from @x.nodes('//*') T(N)
select N.query('.'), N.value('local-name(.)','varchar(max)')+'.' from @x.nodes('/') T(N)

Result1:

Col1                              Col2
<a><b><c /></b><d>eeee</d></a>    a.
<b><c /></b>                      b.
<c />                             c.
<d>eeee</d>                       d.

Result2:

Col1                              Col2
<a><b><c /></b><d>eeee</d></a>    .

---

Answer: (with the help from accepted answer from Mitch)

This query more clearly (relatively speaking) shows what is happening:

declare @t varchar(max) set @t = '<a><b><c/></b><d>eeee</d></a><x />'
declare @x xml set @x = cast(@t as xml)
select N.query('.'), N.value('local-name(.)','varchar(max)')+'.' from @x.nodes('//*') T(N)
select N.query('.'), N.value('local-name(.)','varchar(max)')+'.' from @x.nodes('/*') T(N)
select N.query('.'), N.value('local-name(.)','varchar(max)')+'.' from @x.nodes('/') T(N)

Result1:

Col1                              Col2
<a><b><c /></b><d>eeee</d></a>    a.
<b><c /></b>                      b.
<c />                             c.
<d>eeee</d>                       d.

Result2:

Col1                              Col2
<a><b><c /></b><d>eeee</d></a>    a.
<x />                             x.

Result3:

Col1                                  Col2
<a><b><c /></b><d>eeee</d></a><x />   .

So in the original question, even though the XML in the results 1 and 2 are identical visually, they are "different".

1. The first one is the XML-node starting at taking the (virtual) position of "root"
2. The second one has a "virtual root" that is a "document" element that is not visually displayed.

Answer 1

'/' Selects from the root node

'//' Selects nodes in the document from the current node that match the selection no matter where they are.

Query 1 matches all root elements.

Query 2 matches the single element at the document root. The document root does not have a name.