adding new row to unique table

I have a table with a unique column 'a'. I want to add a row to it.

If the row I want to add has a 'a' value that is already in the table, then of course, due to the uniqueness of the column, it will not be added, else, it will be added. I need to check if the new row was added, which will then tell me if the 'a' value was already in the table or not.

Whats the most efficient way to check if the new entry has been added to the table or not?

EDIT - For measure, the worst case scenario so far is a count be开发者_如何转开发fore and a count after...

I always use ON DUPLICATE KEY UPDATE for that (but I normally want to update rows when the unique key already exits...).

In PDO you can then get a return code with something like $stmt->rowCount() that gives a 1 for a new record and a 2 for an updated record.

I meant something like this (silly example)

$conn = mysql_connect('bla bla bla');
if (!mysql_query("insert into my_table values ('b')")) {
    echo mysql_errno($conn) . ': ' . mysql_error($conn) . "\n";
 }
 else { 
 echo 'ok!';
}