SELECT and INSERT if no row is returned

[Kyong Kim]

I need to INSERT a row into a table only if it does not exist to
insure that there won't be duplicate rows.
Due to the the usage case and the design of the primary key,
non-unique id + auto increment, I don't think insert ignore is an
option.

What would be simplest and cheapest way to make sure that given the
following schema

create table some_table
( parent_id int //non-unique
  seq_id int auto_increment ) ENGINE=INNODB

that no row with the same parent_id can be inserted?

Kyong