mysql从同一个表更新为colum 3条件

Question

这是我的表

kwh       p1 p2 p3 period
2         0  0  0  201408
15        0  0  0  201408
25        0  0  0  201408

我想用这个条件更新p1.p2.p3

UPDATE t.mytable
SET 
    c.p1 = if (c.kwh <= 10,c.kwh * 1600,c.p1),
    c.p2 = if (c.kwh <= 20,c.kwh * 2285,c.p2),
    c.p3 = if (c.kwh => 21,c.kwh - 20 * 3195,p.3),
WHERE
    c.period = '082015'

请帮助