计算MySQL中select的列

Question

假设我有两个表，patients和rooms，而patients是

CREATE TABLE patient (
    id int,
  room int,
  FOREIGN KEY (room) REFERENCES room (id)
);

和room是

CREATE TABLE rooms (
    id int,
);

我想创建一个rooms的视图，其中包括该房间内有多少患者。

我可以计算房间里的病人数量

select count(1) from patients where room = N;

适用于任何现有会议室N。 我如何撰写我需要的SELECT声明？

---

我最好的解决方案：

select *,
       count(1) as patients_in_room
                from patients
                where patients.room = rooms.id
       from rooms;

Answer 1

这将带来所有房间，当patientsCount = 0为空时。

SELECT r.id roomId, coalesce(count(p.room),0) patientsCount
FROM room r left join patients p on r.id = p.room
GROUP BY r.id

Answer 2

SELECT room, COUNT(*) patients_in_room
FROM patient
GROUP BY room

Answer 3

您可以使用MySQL Aggregate函数，请参阅documentation

对于您上面提到的内容，您可以使用以下查询：

SELECT room, COUNT(*) FROM patient GROUP BY room

干杯!!