tsql for xml string concat - How it works

Question

I have the following code which works well:

    STUFF(  ( 
                        select 
                            char(13)+'Item '+i.item+' : '--+char(13) +i.item_descr
                        from @itemlines i
                        where i.customer=main.customer
                        FOR XML PATH(''), TYPE
                    ).value('.','varchar(max)')
                ,1,1, '')

What is the .value() thing? Something like a...select method? What does it do? Any reference links will be appreciated too!

Answer 1

FOR XML will return an XML datatype; the .value(...,...) pulls out the XML value and converts it to the datatype defined. In your case, everything in the root node ('.') converted to varchar(max)

For some blogs/links look at Aaron Bertrand's post or Adam Machanic's also watch out for STRING_AGG a new function in SQL2017

Answer 2

You use FOR XML PATH to convert your table into XML. XML data are represented by a XML data type in SQL Server and it is possible to process a list of different methods on this data type. One of these methods is a value method which has two arguments: XQuery and Data type. The method allows you to convert the data in XML into some other format (varchar in your case).