php - MySQL:加入三个表并连接分组结果

Question

我有以下三个表 'doctors' 、'specialities' 和 'doctor_specialities':

doctors 
-id
-doctor_name

specialities 
-id
-speciality_name

doctor_specialities 
-id
-doctor_id
-speciality_id

我想返回所有医生以及他们的专业名称和专业 ID。一个医生可以有多个专业。

结果集应该是这样的:

id       |       doctor_name       |       speciality_id       |       speciality_name
--------------------------------------------------------------------------------------
1        |       John              |       5,3                 |       Speciality1,Speciality2
3        |       Tim               |       3                   |       Speciality2
6        |       David             |       NULL                |       NULL

我尝试了以下查询:

SELECT d.id ,d.doctor_name, s.speciality_name, s.id
AS speciality_id
FROM api_doctors d
LEFT JOIN api_doctor_specialities ds
ON ds.doctor_id = d.id
LEFT JOIN api_specialities s
ON s.id = ds.speciality_id
GROUP BY d.id

但在这种情况下，我得到的是单一专业。

Answer 1

您正在寻找的是一个名为 GROUP_CONCAT 的 MySQL 函数，它返回一个连接的结果或 NULL。默认分隔符是逗号，因此该查询的结果应该与您想要的结果集相匹配。

SELECT d.id, d.doctor_name, GROUP_CONCAT(s.speciality_name) AS speciality_name, GROUP_CONCAT(s.id) AS speciality_id
FROM api_doctors d
LEFT JOIN api_doctor_specialities ds
ON ds.doctor_id = d.id
LEFT JOIN api_specialities s
ON s.id = ds.speciality_id
GROUP BY d.id

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