MySQL在GROUP BY查询中的值百分比

Question

假设我有3个表： 约会 ， 医生和部门 。

约会：

id     status      doctor_id
----   ---------   ---------
1      approved    1
2      cancelled   4
3      approved    4
4      approved    1
5      approved    4
6      NULL        5
7      approved    2
8      NULL        5
9      approved    4
10     approved    3
11     cancelled   1
12     NULL        4
13     approved    3
14     cancelled   1
15     approved    4
16     cancelled   4
17     cancelled   2
18     NULL        4
19     cancelled   1
20     cancelled   4

医生：

id     name        department_id
----   ---------   -------------
1      John        1
2      Robert      2
3      Patricia    3
4      Mary        1
5      Susan       3

部门：

id     name
----   ---------
1      Dermatology
2      Neurology
3      Radiology

---

我需要的是在皮肤科中按医生分组的已批准任命相对于已批准和已取消（已批准/（已批准+已取消+已取消）* 100）总数的百分比。

我使用以下查询，这使我更接近解决方案。

SELECT COUNT(*) AS appointment_count,
       doctors.name AS doctor_name,
       appointments.status AS appointment_status
FROM appointments
LEFT JOIN doctors ON appointments.doctor_id = doctors.id
LEFT JOIN departments ON doctors.department_id = departments_id
WHERE departments.id = 1
GROUP BY doctors.id,
         appointments.status

结果：

count    doctor_name    appointment_status
-----    -----------    ---------
2        John           approved
3        John           cancelled
0        John           NULL
4        Mary           approved
3        Mary           cancelled
2        Mary           NULL

但是我需要每个医生的批准/（批准+取消）百分比。 因此结果应为：

approved_percentage   doctor_name
-------------------   -----------
%40                   John
%57                   Mary

如何获得这样的结果？

Answer 1

您只能按医生分组，然后使用count（if（））功能，如下所示：

SELECT d.name, count(*), count(if(a.status = 'approved', 1, null)) approved_count,  
    count(if(a.status = 'approved', 1, null))/count(*) * 100 approved_percentage
    FROM doctors d
    INNER JOIN appointments a ON a.doctor_id = d.id
    GROUP BY d.name

Answer 2

在MySQL中，执行条件计数和平均值的最简单方法是简单地使用布尔表达式。 这表明：

SELECT d.name, count(*),
       SUM(a.status = 'approved') as approved_count,  
       AVG(a.status = 'approved') * 100 as approved_percentage
FROM doctors d INNER JOIN
     appointments a
     ON a.doctor_id = d.id
GROUP BY d.name;