Question

我有一个包含USERS（医生和护士），PATIENTS和ASSIGNMENTS表的数据库：

USERS (id, etc.)
PATIENTS (id, doctorid, etc.)
ASSIGNMENTS (id, patientid, doctorid, nurseid, etc.)

当医生登录时，他需要查看他的患者名单以及分配给该患者的NURSE或NURSES。结果应如下所示：

+--------------------+--------------------+-------------------+
| patient first name |  patient last name |  assigned nurses  |
+--------------------+--------------------+-------------------+
| john               |  doe               |  nurse 1, nurse 2 |
| jim                |  jones             |  nurse 7          |
| john               |  doe               |  nurse 4, nurse 5 |
| jim                |  jones             |  nurse 3          |
| jim                |  jones             |  nurse 6          |
+--------------------+--------------------+-------------------+

如何编写sql以返回此数据？

我的sql返回每行的所有NURSES：

SELECT a.id, a.doctorid, a.firstname, a.lastname, GROUP_CONCAT(distinct c.firstname, ' ', c.lastname) as assignednurse

FROM dependents a

        JOIN (
            SELECT ass.id, ass.dependentid, ass.nurseid
            FROM assignments ass
        ) b ON (a.doctorid = 22)

        JOIN (
            u.id, u.doctorid, u.firstname, u.lastname
            FROM users u
            GROUP BY u.id
        ) c ON (c.id = b.nurseid)

        GROUP BY a.id

Answer 1

assignments的加入条件不包含assignments中的字段，因此它实际上是一个跨/笛卡尔联接。此外，您正在进行子查询，而这些子查询似乎并非必要 - 这不应影响返回的值，但可能会降低性能，并会使维护更加困难。相反，尝试：

SELECT a.id, 
       a.doctorid, 
       a.firstname, 
       a.lastname, 
       GROUP_CONCAT(distinct c.firstname, ' ', c.lastname) as assignednurse
FROM patients a
LEFT JOIN assignments b ON a.id = b.patientid
LEFT JOIN users c ON c.id = b.nurseid
WHERE a.doctorid = 22
GROUP BY a.id

MySQL选择子查询或联接

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