查詢幫助

Question

我試圖做一個查詢，查看一張表，看看一個學生是否在一個叫做CMHT的團隊和醫療隊 - 如果他們是我不想看到結果。

我只希望看到記錄，如果他們只在CMHT或軍醫，而不是兩個。

正確的方向是使用子查詢過濾出來嗎？我已經搜索了不在，但你怎麼能看看它是否在2個以上的球隊不是？

Student  Team   ref 
1   CMHT   1 
1   Medic   2 
2   Medic   3 this would be in the result 
3   CMHT   5 this would be in the result 

到目前爲止，我已經做了下面的代碼，我需要使用子查詢或做一個自我加入和過濾它的方式嗎？

SELECT Table1.Student, Table1.Team, Table1.refnumber 
    FROM Table1 
WHERE (((Table1.Team) In ('Medics','CMHT')) 

Answer 1

這是馬克·拜爾斯的用HAVING子句而不是子查詢回答：

SELECT Student, Team, ref 
FROM Table1 
GROUP BY Student 
HAVING COUNT(Student) = 1 

Answer 2

SELECT * 
FROM students 
WHERE NOT EXISTS 
     (
     SELECT NULL 
     FROM students si 
     WHERE si.student = s.student 
       AND si.team = 'CMHT' 
     ) 
     OR NOT EXISTS 
     (
     SELECT NULL 
     FROM students si 
     WHERE si.student = s.student 
       AND si.team = 'Medic' 
     ) 

Answer 3

SELECT a.* 
FROM Table1 a 
INNER JOIN 
(SELECT Student, COUNT(*) FROM Table1 
GROUP BY Student 
HAVING COUNT(*) = 1)b 
ON (a.Student = b.Student) 

Answer 4

如何，你可以在這裏看到檢查其在2個人以上的團隊？

你可以指望每個學生團隊的數量，然後篩選只有那些你希望看到：

SELECT student FROM 
(
    SELECT student, COUNT(*) AS cnt 
    FROM Table1 
    GROUP BY student 
) T1 
WHERE cnt = 1 

Answer 5

你可以用外做加盟

select COALESCE(t1.Student, t2.Student) as Student, 
     COALESCE(t1.Team, t2.Team) as Team, 
     COALESCE(t1.ref, t2.ref) as ref 
from 
    (select * from Student where Team = 'CMHT') t1 
    outer join 
    (select * from Student where Team = 'Medic') t2 
    on t1.Student = t2.Student 
where 
    t1.Student is null or 
    t2.Student is null;