Reason for resolution:

In the process of multi-level query, you need to give the table alias.

Code example

Wrong query method:

select * from (select s.sno from student s where s.ssex='woman');

Correct query method:

select * from (select s.sno from student s where s.ssex='woman') as mid_sno;

In fact, the difference lies in whether there is an as mid behind it_sno. This is to ensure that each derived table needs to have its own alias.