多个嵌套级别查询的MySQL 1054错误

Question

我对MySQL 5.6的跟随查询有一个问题：

select 
(c.datefield) as dfield,
(select 
    count(t1.C_URI)
    from 
        (select C_URI,MAX(C_updated) dataOpen from  workitem wi where C_status<>'closed' and date(C_updated)<=c.datefield group by C_URI) t1
    left outer join
        (select C_URI,MAX(C_updated) dataClosed from  workitem wi where C_status='closed' and date(C_updated)<=c.datefield group by C_URI) t2   
    on 
        t1.C_URI=t2.C_URI
    where 
        t1.dataOpen> ifnull(t2.dataClosed,0)
) as tot
from 
    calendar c 
left join 
    workitem 
on 
    date(datefield)=date(C_updated)
where
    datefield between ('2014-10-01') and ('2014-10-20')
group by 
    datefield;

问题是MySQL无法识别date(C_updated)<=c.datefield引发错误：

Error Code: 1054. Unknown column 'c.datefield' in 'where clause'    0.000 sec

我该如何解决这个问题？

提前谢谢你......！