INSERT or UPDATE - Mailing list pgsql-sql

Hello

I have an XML file, which i must import into the database. I transform
this XML File with xslt to sql INSERTS.

But there is also the possibility, that one row is available. Means that
i need an UPDATE instead of the INSERT.

Is it in SQL possible to decide if there is an UPDATE or an INSERT or
must i program something with PHP for example?

Thanks
Andreas