Comment utiliser ON DUPLICATE KEY UPDATE dans MySQL sans index UNIQUE ou PRIMARY KEY ?

INSERT INTO `table` (value1, value2) 
SELECT 'stuff for value1', 'stuff for value2' FROM `table` 
WHERE NOT EXISTS (SELECT * FROM `table` 
                  WHERE value1='stuff for value1' AND value2='stuff for value2') 
LIMIT 1

Source :Comment 'insérer s'il n'existe pas' ' dans MySQL ?