My site has a messaging feature where one user may message another. The messages support threading - a parent message may have any number of children but only one level deep.

The messages table looks like this:

Messages
 - Id (PK, Auto-increment int)
 - UserId (FK, Users.Id)
 - FromUserId (FK, Users.Id)
 - ParentMessageId (FK to Messages.Id)
 - MessageText (varchar 200)

I'd like to show messages on a page with each 'parent' message followed by a collapsed view of the children messages.

Can I use the GROUP BY clause or similar construct to retrieve parent messages and children messages all in one query? Right now I am retrieving parent messages only, then looping through them and performing another query for each to get all related children messages.

I'd like to get messages like this:

Parent1
 Child1
 Child2
 Child3
Parent2
 Child1
Parent3
 Child1
 Child2