在SQL中查詢子查詢結果

Question

我對SQL比較陌生，可能會想得不明白，但是給出了下表：我想要列出所有已注冊化學而不是數學的學生的清單，因此給定數據，我將擁有學生1。

    Student_ID  Subject
      1      Chemistry
      2      Mathematics
      2      Chemistry
      3      History

這是我嘗試過的

SELECT        Student_ID
FROM          Student 
WHERE        (Subject = 'Chemistry') AND (Subject <> 'Mathematics')

也許我需要將其分組，因為這兩個條件都存在行和返回值。

Answer 1

這是使用conditional aggregation的一種選擇：

select student_id
from student
group by student_id
having max(case when subject = 'Chemistry' then 1 end) = 1 and
       max(case when subject = 'Mathematics' then 1 else 0 end) != 1

---

這是另一個not exists的選項：

select student_id
from student s
where subject = 'Chemistry' and not exists (
    select 1
    from student s2
    where s.student_id = s2.student_id and st.subject = 'Mathematics'
    )

Answer 2

您可以使用where而不是in

select Student_ID
from Student 
where Subject  = 'Chemistry'
and Student_ID NOT IN  ( select Student_ID from Student where subject ='Mathematics');