Concatenate all values of the same XML element using XPath/XQuery
This might work for you:

select @MyXml.value('/R[1]', 'varchar(50)')


It picks up all text() elements from the first R and below.

If you just want all text() you can do 

select @MyXml.value('.', 'varchar(50)')


If you want the values for I and J separate do this instead.

select @MyXml.query('/R/I/text()').value('.', 'varchar(50)'),
       @MyXml.query('/R/J/text()').value('.', 'varchar(50)')