计算顺序不是先where后select吗？为什么能用select计算出别名？
select distinct item_id,userbehavior.item_category,amount
from userbehavior
inner join (select item_category,max(amount) as 最高工资 from userbehavior group by item_category) as t
on userbehavior.item_category=t.item_category
where amount=最高工资;