QUESTION: Automatically generating Primary keys !!!

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Hello,

I'm working on some sort of search-engine which is accesable by a browser.
On of the features of the engine is that you can insert new data into it
just by using your browser. For all this I use PHP, my database is in
Postgresql.

My problem is that I should be able to let the Database System (Postgresql)
generate its own primairy keys (from the rest of the data).

eg. I've got a table like this:

workplace_ID, name, street, nr, zipcode, city, country

workplace_ID is the primairy key of that table. Some other tables have a
foreign key refering to it. As a primairy key this ID is of course unique.
My Question is: How can I see which values (of the primairy key) are already
taken, and which is available for new data.

Thanks a lot

STom

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