Question

我有两个问题：

SELECT  city, count(id) as num_of_applicants 
FROM(
    select distinct(students.id), city
    FROM STUDENTS INNER JOIN APPLICATIONS ON STUDENTS.ID = APPLICATIONS.STUDENT_ID
    WHERE APPLICATIONS.COLLEGE_ID = '28'
    ) AS derivedTable
GROUP BY city;

SELECT city, count(id) as num_of_accepted_applicants
 FROM
     (select applications.id, city FROM
     STUDENTS INNER JOIN APPLICATIONS ON STUDENTS.ID = APPLICATIONS.STUDENT_ID
     WHERE status = 'Accepted' and college_id = '28') as tbl
GROUP BY city

一个给每个大学的申请人数量，一个给出每个大学的被接受申请人数，但是我想得到一个结果在查询（而不是）结果是这样的结果：
city | number_of_applicants | number_of_accepted_applicants

Answer 1

你可以简化（ fyi：我不明白你为什么使用派生表，你可以把COUNT和GROUP BY放在内部查询上）并将这些查询组合成：

SELECT city
  , COUNT(*) AS num_of_applicants
  , SUM( CASE
            WHEN status = 'Accepted' THEN 1
            ELSE 0
         END
       ) AS num_of_accepted_applicants
FROM STUDENTS
JOIN APPLICATIONS 
  ON STUDENTS.ID = APPLICATIONS.STUDENT_ID
WHERE college_id='28'
GROUP BY city;

另一种方法是继续使用派生表的技术。将您的每个查询设为派生表并加入城市 - 但这样做也不会有效。

在PostgreSQL中按多列分组

1 个答案: