How to immediately get the AUTO_INCREMENT id of a duplicate record on INSERT?

Question

MySQL

A frequent case when you need to insert a record, and if there is already a record with such a UNIQ key, then return the AUTO_INCREMENT ID field for this record.
Now I'm doing it in the forehead:

if(mysql_query("INSERT INTO tab (hash) VALUE ('hash')"))
  {
  if(mysql_errno()== ER_DUP_KEY)
    {
    mysql_query("SELECT id FROM tab WHERE hash = 'hash'");
    }
  }

But in theory, the server inside has already felt this entry and knows where it is :)
ps INSERT ... ON DUPLICATE KEY UPDATE unfortunately will not return an AUTO_INCREMENT column for the case with UPDATE

Answer 1

Let's say this:

INSERT INTO table (a,b,c) VALUES (1,2,3)
  ON DUPLICATE KEY UPDATE id=LAST_INSERT_ID(id), c=3;

Answer 2

Holmes, but how? :)
It works, but I don't understand.
The only drawback is that each hit in a duplicate generates INCREMENT, I'll try to test which is cheaper