解决错误操作数时遇到问题应包含 1 列

Question

我正在尝试选择班级中失败学生最多的老师的姓名。

select 
    Failed.section, max(Failed) as Failed_Class 
from 
    (select 
         e.section, count(e.grade) * 100 / (select count(*)
                                            from enrollment) as Failed
     from 
         enrollment e 
     where 
         e.grade = 'D' or e.grade = 'F'
     group by 
         e.section) as Failed;

这将返回部分 id 和该部分中不及格学生的百分比。

select 
    p.name
from 
    professors p, teaches t 
where 
    p.ssn = t.ssn 
    and t.section in (select Failed.section, max(Failed) as Failed_Class 
                      from 
                          (select e.section, count(e.grade) * 100 / (select count(*) from enrollment) as Failed
                           from enrollment e 
                           where e.grade = 'D' or e.grade = 'F'
                           group by e.section) as Failed);

当我执行 sub select 语句时，我得到了错误，我不知道如何解决这个问题？

我得到的错误是：

操作数应包含 1 列

Answer 1

该错误与您使用返回两列并尝试与一列的 in 子句匹配的子查询有关。

另外，如果您只需要一位老师，您可以使用 order by max value desc 和 limit 1

select p.name
from professors p 
inner join teaches t ON p.ssn = t.ssn 
  and t.section = t.section in (
  select Failed.section
  from (
    select e.section 
    ,count(e.grade)*100/ ( select count(*)  from enrollment ) as Failed
  from enrollment e 
  where e.grade = 'D' or e.grade = 'F'
  group by e.section
  ORDER BY failed 
  limit 1 
  ) t 
);

最后，您应该使用显式连接语法（而不是旧的基于位置的隐式连接语法）。