One quick solution to your problem is using a "recursive" structure in your POSTS table.

POSTS:
PID
ParentPID
UserID

Basically, when someone new replies to a post, the original first PID should be inserted into the ParentPID column.

Example of how this would look:

PID = 1000
ParentPID = NULL (or 0)
UserID = 100

PID=1001
ParentPID = 1000
UserID = 99

Then all you have to do to show the original answer (first), is to use PARENTPID IS NULL.

And then order the PARENTPID thereafter...
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Buh Bye!

Cheers,
Me