sql - What is the meaning of SELECT ... FOR XML PATH(' '),1,1)?

Question

I am learning sql in one of the question and here I saw usage of this,can some body make me understand what xml path('') mean in sql? and yes,i browsed through web pages I didn't understand it quite well!

I am not getting the Stuff behind,now what does this piece of code do ?(only select part)

declare @t table
(
    Id int,
    Name varchar(10)
)
insert into @t
select 1,'a' union all
select 1,'b' union all
select 2,'c' union all
select 2,'d' 

select ID,
stuff(
(
    select ','+ [Name] from @t where Id = t.Id for XML path('')
),1,1,'') 
from (select distinct ID from @t )t

Answer 1

There's no real technique to learn here. It's just a cute trick to concatenate multiple rows of data into a single string. It's more a quirky use of a feature than an intended use of the XML formatting feature.

SELECT ',' + ColumnName ... FOR XML PATH('')

generates a set of comma separated values, based on combining multiple rows of data from the ColumnName column. It will produce a value like ,abc,def,ghi,jkl.

STUFF(...,1,1,'')

Is then used to remove the leading comma that the previous trick generated, see STUFF for details about its parameters.

(Strangely, a lot of people tend to refer to this method of generating a comma separated set of values as "the STUFF method" despite the STUFF only being responsible for a final bit of trimming)