An alternative that doesn't use a second table or make you
increment numbers for a CASE statement:
SELECT
*
FROM
products
WHERE
id IN (13,3,2,5,9,12,7)
ORDER BY
id=13 DESC,
id=3 DESC,
id=2 DESC,
id=5 DESC,
id=9 DESC,
id=12 DESC,
id=7 DESC
;