mysql automatically prevent redundant inserts -

i have table like

create table adjacencies(   relationid int not null primary key auto_increment,   parent int not null,   child int not null,   pathlen int not null ) ; 

i'm inserting large amounts of entries like

insert adjacencies( parent, child, pathlen ) select a, b, c condition ; 

so when run insert query, possible (parent,child) relation exist , duplicated. bad news.

i don't want create primary key on ( parent, child ) because results in hard fail (query failure).

what want soft fail if there attempt insert (parent, child) pair exists in table (i.e pair gets ignored, , rest of query proceeds normally).

what's best way in mysql?

(* wondering why there relationid member in table?)

create composite unique key around parent , child columns. issue query instead:

insert ignore adjacencies( parent, child, pathlen ) select a, b, c condition ; 

the ignore clause turn duplicate key errors warnings.