problem solved

["pavka14" <pavka14@yahoo.com>]

Thank you very much for the informative answer :-)

In the meanwhile though, our programmers found a different solution.
It also relies on an external source to guarantee uniqueness, but
that's a separate table in the database.
This second table has only the columns that need to be unique, and
they are indexed as such.
Then, when a new row has to be insterted, it's tried as one
transaction simultaneously in the two databases. If it bounces in the
second (artifitial one), then the query in the real database is rolled
back.
This is all on an Oracle database.

I am not a great programmer myself, I just do php/MySQL as a hobby.
And our programmers here don't speak much English, so I was sort uf
asking on their behalf.

Thanks again for the assistance.