Come aggiorno se esiste, inserisco in caso contrario (AKA upsert o merge) in MySQL?

Usa INSERT ... ON DUPLICATE KEY UPDATE . Ad esempio:

INSERT INTO `usage`
(`thing_id`, `times_used`, `first_time_used`)
VALUES
(4815162342, 1, NOW())
ON DUPLICATE KEY UPDATE
`times_used` = `times_used` + 1