Объединить вывод различных запросов в виде одного запроса

Question

Как мне объединить вывод разных запросов.

У меня есть следующие sql запросы с выводом:

1) select d.departmentid, d.name, count(distinct(sg.personid)) as noOfStudents from department d inner join course c on c.departmentid = d.departmentid inner join student_grade sg on sg.courseid = c.courseid group by d.departmentid, d.name;
+--------------+--------------+--------------+
| departmentid | name         | noofstudents |
+==============+==============+==============+
|          101 | Computer Sci |            1 |
|          104 | Mech         |            2 |
|          103 | EEE          |            1 |
+--------------+--------------+--------------+



2) select d.departmentid, d.name, count(distinct(ci.personid)) as noOfTeachers from department d inner join course c on c.departmentid = d.departmentid inner join course_instructor ci on ci.courseid = c.courseid group by d.departmentid, d.name;
+--------------+--------------+--------------+
| departmentid | name         | noofteachers |
+==============+==============+==============+
|          101 | Computer Sci |            1 |
|          103 | EEE          |            2 |
|          104 | Mech         |            1 |
|          102 | ECE          |            3 |
+--------------+--------------+--------------+

3) select d.departmentid, d.name, count(distinct(c.courseid)) as noOfCourses from department d inner join course c on c.departmentid = d.departmentid group by d.departmentid, d.name;
+--------------+--------------+-------------+
| departmentid | name         | noofcourses |
+==============+==============+=============+
|          101 | Computer Sci |           3 |
|          102 | ECE          |           3 |
|          104 | Mech         |           1 |
|          103 | EEE          |           2 |
+--------------+--------------+-------------+

Теперь я хочу объединить все три в одну таблицу для отображения данных. Как я могу это сделать?

Здесь я пробую операцию UNION, и это хорошо использовать?

Answer 1

Если вам нужно получить в качестве отдельных столбцов в выходных данных студентов, учителей и курсы, вы можете попробовать что-то вроде этого:

select 
    d.departmentid, d.name, 
    count(distinct(sg.personid)) as noOfStudents, 
    0 as noofteachers, 0 as noofcourses 
from 
    department d 
inner join 
    course c on c.departmentid = d.departmentid 
inner join 
    student_grade sg on sg.courseid = c.courseid 
group by 
    d.departmentid, d.name

UNION ALL

select 
    d.departmentid, d.name, 0 as noOfStudents,
    count(distinct(ci.personid)) as noOfTeachers, 0 as noofcourses 
from 
    department d 
inner join 
    course c on c.departmentid = d.departmentid 
inner join 
    course_instructor ci on ci.courseid = c.courseid 
group by 
    d.departmentid, d.name

UNION ALL

select 
    d.departmentid, d.name, 0 as noOfStudents, 
    0 as noofteachers, count(distinct(c.courseid)) as noOfCourses 
from 
    department d 
inner join 
    course c on c.departmentid = d.departmentid 
group by 
    d.departmentid, d.name

Это должно произвести что-то вроде этого:

+--------------+--------------+--------------+--------------+--------------+  
| departmentid | name         | noofstudents | noofteachers | noofcourses  |  
+==============+==============+==============+==============+==============+  
|          101 | Computer Sci |             1|             0|             0|  
|          104 | Mech         |             2|             0|             0|  
|          103 | EEE          |             1|             0|             0|  
|          101 | Computer Sci |             0|             1|             0|  
|          103 | EEE          |             0|             2|             0|  
|          104 | Mech         |             0|             1|             0|  
|          102 | ECE          |             0|             3|             0|  
|          101 | Computer Sci |             0|             0|             3|  
|          102 | ECE          |             0|             0|             3|  
|          104 | Mech         |             0|             0|             1|  
|          103 | EEE          |             0|             0|             2|  
+--------------+--------------+--------------+--------------+--------------+  

Если вам нужны агрегированные значения, просто заключите весь запрос в скобки и суммируйте результаты

SELECT 
    departmentid, name, 
    SUM(noOfStudents), SUM(noOfTeachers), SUM(noOfCourses)
FROM 
    (Query1 UNION ALL Query2 UNION ALL Query3) AS mySubQuery
GROUP BY 
    departmentid, name

Answer 2

Вы можете использовать внутреннее соединение для получения всех результатов в одной строке

  select  t0.departmentid
        , t0.name
        , t1.noOfStudents
        , t2.noOfTeachers 
        , t3.noOfCourses 
  from department t0
  left join 
  (
  select d.departmentid
    , d.name
    , count(distinct(sg.personid)) as noOfStudents
  from department d 
  inner join course c on c.departmentid = d.departmentid 
  inner join student_grade sg on sg.courseid = c.courseid 
  group by d.departmentid, d.name
  ) t1 ON t0.departmentid = t1.departmentid
  left join (
    select d.departmentid
      , d.name
      , count(distinct(ci.personid)) as noOfTeachers 
    from department d 
    inner join course c on c.departmentid = d.departmentid 
    inner join course_instructor ci on ci.courseid = c.courseid 
    group by d.departmentid, d.name
  ) t2 on t0.departmentid = t2.departmentid 
  left join  (
    select d.departmentid
    , d.name
    , count(distinct(c.courseid)) as noOfCourses 
    from department d 
    inner join course c on c.departmentid = d.departmentid 
    group by d.departmentid, d.name
  ) t3 on t0.departmentid = t3.departmentid 

или если вам нужен результат на отдельных строках, вы можете использовать UNION ALL