Is it possible to insert a row but only if a value does not already exist?

Question

Is it possible to insert a row, but only if one of the values already in the table does not exist?

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Answer 1

I'm not sure if I got it, but what about a

try {
  mysql_query($sql);
}
catch(Exception $e) {

}

combined with an unique field index in MySQL?

if it throws an exception then you know that you got a duplicated field. Sorry if that don't answer your question..