Stuff with XML PATH

Question

id Email

1 [email protected]

1 [email protected]

2 [email protected]

3 [email protected]

3 [email protected]

 

output:

id email

1 [email protected],[email protected]

2 [email protected]

3 [email protected],[email protected]

 

I found the following query some where can you explain it?

 

SELECT id, Email =

STUFF((SELECT ', ' + Email

FROM comma b

WHERE b.id = a.id

FOR XML PATH('')), 1, 1, '')

FROM comma a

GROUP BY id