MySQL INSERT if not exist (do not insert if exist)

Question:
How can I insert into a row if the pair does not exist?

Answer:
To get the solution, we have just to follow the below simple steps.

1) Can you add a UNIQUE constraint on (myid, theirid)? If yes, add this constraint and use:

INSERT INTO mytable (myid, theirid) 
  VALUES (5, 1) ;

and ignore the produce warnings (or replace the above with INSERT IGNORE)

2) If you can't add such a constraint (e.g. you sometimes want to allow such duplicates and other times you don't), you can use this:

INSERT INTO mytable (myid, theirid) 
SELECT 5, 1 
FROM dual 
WHERE NOT EXISTS
 ( SELECT *
   FROM mytable
   WHERE myid = 5
  AND theirid = 1
 ) ;