Interpoler manglende værdier i en MySQL-tabel

Dette burde virke:

SET @prev = NULL;

UPDATE   ticks
SET      quote= @prev := coalesce(quote, @prev)
ORDER BY tick;

BTW det samme trick virker til at læse:

SELECT   t.tick, @prev := coalesce(t.quote, @prev)
FROM     ticks t
JOIN     (SELECT @prev:=NULL) as x -- initializes @prev
ORDER BY tick