MySQL:仅在满足条件时更新字段

Is it possible to do an UPDATE query in MySQL which updates field value only if certain condition is met? Something like this:

UPDATE test
SET
    CASE
        WHEN true
        THEN field = 1
    END
WHERE id = 123

换句话说:

UPDATE test
SET
    something = 1,        /*field that always gets updated*/
    CASE
        WHEN true
        THEN field = 1    /*field that should only get updated when condition is met*/
    END
WHERE id = 123

这样做的正确方法是什么?

UPDATE prices
SET final_price= CASE
   WHEN currency=1 THEN 0.81*final_price
   ELSE final_price
END