isnull or coalesce isn't working

HI
just started using postgres I want to do a select like the following.
I have a table with two columns a and b, in b sometimes data are missing.
I want to select column b, and if data are missing instead column a
as one column in my select.
I did:
SELECT ifnull(a,b) FROM table;

==>ERROR: No such function 'ifnull' with the specified attributes

tried the same thing with isnull and coalesce, same error.
(on my machine: postgresql version 6.4.2)

help?! thanks, jochen

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