MySQL中case then用法

1.查询图书价格，若价格为null，则显示unknown，若价格为10到20， 则显示10 to 20

SELECT price,
CASE
WHEN price='null' THEN 'UnKnow'
WHEN price>10 and price<20 then '10 to 20'
END
FROM book;

2.

select name,
 case
        when birthday<'1981' then 'old'
        when birthday>'1988' then 'yong'
        else 'ok' END YORN
from lee;

3.

select NAME,
 case name
     when 'sam' then 'yong'
        when 'lee' then 'handsome'
        else 'good' end
from lee;