sql - 如何解决postgresql中group by和aggregate函数的问题

Question

我正在尝试编写一个查询来划分这两个 SQL 语句，但它显示了我

 ERROR: column "temp.missed" must appear in the GROUP BY clause or be used in 
 an aggregate function
  SQL state: 42803

虽然当我按 temp.missed 进行分组时，它似乎有效但由于多个分组依据而显示错误的结果。

我在 PostgreSQL 中有以下表格

 create table test.appointments (
      appointment_id serial
      , patient_id integer references test.patients(id)
      , doctor_id integer references test.doctors(id)
      , appointment_time timestamp
      , appointment_status enum('scheduled','completed','missed')
      );

 create table test.visits (
      visit_id serial
      , patient_id integer references test.patients(id)
      , doctor_id integer references test.doctors(id)
      , walk_in_visit boolean
      , arrival_time timestamp
      , seen_time timestamp
      );

我已经编写了以下查询来查找错过的约会率(错过的约会/总约会)，但它显示了上述错误。

  select tem.doctor_id, (temp.missed::float/tem.total) as ratio from 
    (select doctor_id, count(appointment_status) as missed from appointments 
     where appointment_status='missed' group by doctor_id)as temp
      join (select doctor_id, count(appointment_status) as total from 
      appointments group by doctor_id) as tem  on temp.doctor_id = 
       tem.doctor_id group by tem.doctor_id;

Answer 1

您不需要所有这些子查询 - 应用于 case 表达式的 count 函数会简单得多:

SELECT   doctor_id, 
         COUNT(CASE appointment_status 
                    WHEN 'missed' THEN 1 
                    ELSE NULL 
               END)::float / 
         COUNT(*) AS ratio
FROM     appointments
GROUP BY doctor_id