Auomatic numbering, replacing nulls and IF

I'm a child of Oracle and MS-SQL (forgive me) and have only been playing
with Postgres for a few days. I have 3 questions that I couldn't find
answers for in the online docs:

1) Is it possible to create an INT PRIMARY KEY column that automagically
numbers itself? MS-SQL does this using the IDENTITY keyword.

2) The ISNULL function in both Oracle and MS-SQL takes 2 numbers and returns
the second if the first is null, otherwise it returns the first. ISNULL in
Postgres seems to be something quite different. Before I go writing my own
function to do it, is there one already?

3) How do I do 'IF this_sql THEN that_sql'? I keep getting:
parse error at or near "IF"
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Luke Pascoe
Senior Developer
IT Support & Development Limited
http://wwwitsd.co.nz
Mobile: (021) 303019
Email: luke(at)itsd(dot)co(dot)nz
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