總結(jié)一下：兩種表達(dá)方式是等效的
方法一：

case 
when tb1.os = 'android' then 'android'
when tb1.os = 'ios' then 'iPhone'
else 'PC'
end as os,

方法二：

case tb1.os
when 'android' then 'android'
when 'ios' then 'iPhone'
else 'PC'
end as os,

完整示例：

select user_id,
case location
when '北京' then '1'
else null
end as location_in_beijing
from employees