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:

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1 2	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:

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