Re: SQL( "if ...exists...),how to do it in the PostgreSQL?

>> if not exists(select id from test) insert into test(id) values (280);

> Write a function in PL/pgSQL.

That particular case could be handled like so:

insert into test(id) select 280 where not exists(select id from test);

The select produces either zero or one row depending on whether its
WHERE is true.  Voila, problem solved.  It's even nearly standard ;-)
although in something like Oracle you'd have to add "from dual", I
think.
        regards, tom lane