MySQL query question

[Jack]

ya, it's phpBB... but is really just because i'm not that great as SQL.

anyways...

a[100, 101, 102, 103, 104] b[3, 8, 3, 1, 8]

the a&b are the fields and 100 -> 3 ; 101 -> 8 ; ... ; 104 -> 8

(that's my best way of making a table)

now, for the output, I want: 1, 3, 8

yet, I want them in the order of 8, 1, 3

as you can see, I want the numbers sorted by the 'a' field DESC but I don't want repeats in the b field.

actually what i'm trying to do is (you better know the phpBB table layout) is select the topic_id from phpbb_posts sorted by post_time with unique (no repeats) of the topic_id.

can anyone help?

PS: DISTINCT, DISTINCT ROW whatevery you like.