连接2个表以返回结果为零的查询

Question

我有2张桌子，一张叫医生，一张叫做约会。他们俩都有着共同关系的医生。我需要能够生成医生ID列表以及为每位医生预约的次数，无需预约。 （参见表格内容的图片）

使用下面的代码我可以获得医生名单id和约会数量，但它不会显示医生列表中的ZERO约会医生。我需要做哪些修改？

SELECT doctor_id, COUNT(DISTINCT appt_time) AS No_APP FROM appointment GROUP BY doctor_id;

这是我从中得到的

Answer 1

尝试：

SELECT doctor.doctor_id, COUNT(appointment.appt_time) AS No_APP 
FROM doctor
LEFT JOIN appointment 
ON appointment.doctor_id = doctor.doctor_id
GROUP BY doctor.doctor_id;

Answer 2

使用左连接：

SELECT doctor.doctor_id, COUNT(appt_time) AS No_APP FROM doctor LEFT JOIN appointment ON doctor.doctor_id = appointment.doctor_id GROUP BY doctor.doctor_id;

它应该做的伎俩

Answer 3

这应该有效：

select 
  d.doctor_id
  ,count(distinct a.appt_time) as No_APP 
from doctor d 
left join appointment a on d.doctor_id=a.doctor_id
group by d.doctor_id

Answer 4

使用左连接：

SELECT doctor.doctor_id, COUNT(DISTINCT appt_date, appt_time) AS No_APP 
FROM doctor 
LEFT JOIN appointment
ON doctor.doctor_id = appointment.doctor_id
GROUP BY doctor_id;