SQL 子查詢未返回預期結果

Question

Select name 
from instructor 
where dept_name = 'Comp. Sci.' and ID in
    (select ID
     from course 
     where title != 'Intro. to Computer Science')

有了這個輸入，我希望輸出是從未教過“計算機科學導論”的教師，但輸出是來自 CS 的所有教師，而不僅僅是沒有教過計算機科學導論的 1 位教授 Brant。我做錯了什么？

Answer 1

這應該適合你

select i.name 
from instructor as i, teaches as t
where i.dept_name = 'Comp. Sci.' and i.ID=t.ID
and not exists (
  select 1 
  from course 
  where t.ID = course.course_id and title = 'Intro. to Computer Science'
)

Answer 2

使用不存在：

select i.name 
from instructor i
where i.dept_name = 'Comp. Sci.' 
and not exists (
  select 1 
  from teaches t inner join course c
  on c.course_id = t.course_id 
  where t.id = i.id and c.title = 'Intro. to Computer Science'
)

或按教師分組並在HAVING子句中設置條件：

select i.name 
from instructor i
inner join teaches t on t.id = i.id
inner join course c on c.course_id = t.course_id 
where i.dept_name = 'Comp. Sci.'
group by i.id, i.name
having count(case when c.title = 'Intro. to Computer Science' then 1 end) = 0