查询帮助

Question

我试图做一个查询，查看一张表，看看一个学生是否在一个叫做CMHT的团队和医疗队 - 如果他们是我不想看到结果。

我只希望看到记录，如果他们只在CMHT或军医，而不是两个。

正确的方向是使用子查询过滤出来吗？我已经搜索了不在，但你怎么能看看它是否在2个以上的球队不是？

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 

到目前为止，我已经做了下面的代码，我需要使用子查询或做一个自我加入和过滤它的方式吗？

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

Answer 1

这是马克·拜尔斯的用HAVING子句而不是子查询回答：

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，你可以在这里看到检查其在2个人以上的团队？

你可以指望每个学生团队的数量，然后筛选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;