Selecting exactly one row for each column value

I've got the following table:

fweimer=> SELECT * FROM tab;
a | b | c
---+---+---
1 | 2 | 3
5 | 6 | 7
1 | 2 | 2
2 | 3 | 4
1 | 2 | 2
2 | 3 | 4

For each value in the first column, I need one (and only one) matching
row from the table. A possible solution is:

a | b | c
---+---+---
5 | 6 | 7
2 | 3 | 4
1 | 2 | 3

Of course,

SELECT a, (SELECT b FROM tab i WHERE i.a = o.a LIMIT 1),
(SELECT c FROM TAB i WHERE i.a = o.a LIMIT 1) FROM tab o GROUP BY o.a;

does the trick, but this approach seems to rely on undefined behavior
and quickly gets messy when the number of columns increases.

Is there a better way to implement this?

--
Florian Weimer <fweimer(at)bfk(dot)de>
BFK edv-consulting GmbH http://www.bfk.de/
Kriegsstraße 100 tel: +49-721-96201-1
D-76133 Karlsruhe fax: +49-721-96201-99