question about a select

OK, I have the following table:

create table citations_by_level
(
aid smallint,
wid smallint,
v_level varchar(50),
w_level varchar(50),
x_level varchar(50),
y_level varchar(50),
z_level varchar(50),
byteloc integer
);

(If it helps, aid/wid identifies a text or work, the levels are
citation levels for that work (all but z_level potentially optional);
eg for some work y_level might indicate chapters [z_levels indicate
lines], and byteloc is the file position of that particular citation
in the work).

What I would *like* to be able to do is construct a query that groups
by a level, but sorts by byteloc. I don't seem to be able to do this.

Here are some examples. Note that y_level (any level) may have
duplicates (which I want to eliminate), and that it's ordering is
strictly on byteloc, not on its own value. postgres doesn't seem to
have envisioned this scenario and/or I'm not being creative enough in
constructing the query...

Text=# select distinct on (y_level) y_level from citations_by_level where aid=543 and wid=1;
y_level
---------
1
10
10a
11
12
13
14
15
16
17
18
19
2
20
21
22
23
24
25
26
27
28
29
2a
3
30
31
32
33
34
35
36
37
4
5
5a
6
7
7,8
8
9
t
(42 rows)

but as you can see, the "ordering" winds up being alphabetic on
y_level which simply does not do. [In this case it is only
coincidental that y_level appears numeric, it is a string and could be
anything; and the 7,8 is such an example]. The *byteloc* associated
with a given y_level (the location of that particular citation) does,
but I can't seem to use it:

Text=# select distinct on (y_level) y_level, byteloc from citations_by_level where aid=543 and wid=1 order by byteloc;
ERROR: SELECT DISTINCT ON expressions must match initial ORDER BY expressions

Text=# select distinct on (y_level) y_level, byteloc from citations_by_level where aid=543 and wid=1 order by byteloc, y_level;
ERROR: SELECT DISTINCT ON expressions must match initial ORDER BY expressions

???

--Cindy
--
ctmoore(at)uci(dot)edu