具有自联接访问权限的子查询，无法识别表别名

Question

I built some subquery:

select 
    q2.addedquests, q2.daynum
from 
    qrNumberOfQuestsToDo as q2 
inner join 
    qrNumberOfQuestsToDo as q3 on q2.daynum > q3.daynum
where 
    q2.DayNum = (select max(q3.DayNum) from q3);

but MS Access does not recognize q3 in the subquery. Why?

Answer 1

Why not? Because that is how SQL works. You can refer to a column of q3 in the subquery (in the select , where , group by , having , order by , or on clause for instance), but not to the entire table.

I am continuing, but I think the query is non-sensical. In one place, the query says that q2.daynum > q3.daynum . In another, that q2.daynum = q3.daynum . Hence, the query will not return anything. But, you can still express it as valid SQL. Notice that q3 is not really needed in the outer query. Try a correlated subquery instead:

select q2.addedquests, q2.daynum
from qrNumberOfQuestsToDo as q2 
where q2.DayNum = (select max(q3.DayNum)
                   from qrNumberOfQuestsToDo q3
                   where q2.daynum>q3.daynum
                  )  ;