帮助查询

Question

我正在尝试进行查询以查看单个表，以查看学生是否在名为 CMHT 的团队和医疗团队中 - 如果他们是，我不想看到结果。

我只想查看他们仅在 CMHT 或 medic 中的记录，而不是两者。

正确的方向是使用子查询来过滤掉它吗？我已经对 NOT IN 进行了搜索，但你怎么能看到它是否在超过 2 个团队中？

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，我已经完成了以下代码，我需要使用子查询还是进行自联接并以这种方式过滤？

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

Answer 1

这是 Mark Byers 用 HAVING 子句代替子查询的答案：

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

您如何查看它是否在 2 个或更多团队中？

您可以计算每个学生的团队数量，然后只过滤您想查看的团队：

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

Answer 5

你可以通过外部连接来做到这一点

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;