sql 超过 2 个表的内连接和聚合函数

Question

我有以下表格，

select * from department;
+--------------+--------------+--------+------------+---------------+
| departmentid | name         | budget | startdate  | administrator |
+==============+==============+========+============+===============+
|          101 | Computer Sci |   1000 | 2010-12-26 | XYZ           |
|          102 | ECE          |    500 | 2015-02-15 | ABC           |
|          103 | EEE          |   1500 | 2016-08-25 | PQR           |
|          104 | Mech         |   2500 | 2017-08-22 | LMN           |
+--------------+--------------+--------+------------+---------------+

select * from course;
+----------+-------------------+---------+--------------+
| courseid | title             | credits | departmentid |
+==========+===================+=========+==============+
|     1001 | Data structures   |      12 |          101 |
|     1002 | Algorithms        |      12 |          101 |
|     1003 | Graphics          |      20 |          101 |
|     2001 | DSP               |      20 |          102 |
|     2002 | Matlab            |      20 |          102 |
|     2003 | Maths             |      10 |          102 |
|     3001 | CAD               |      10 |          104 |
|     4001 | Power electronics |      10 |          103 |
|     4002 | Semi conductors   |      20 |          103 |
+----------+-------------------+---------+--------------+

select * from student_grade;
+--------------+----------+----------+-------+
| enrollmentid | courseid | personid | grade |
+==============+==========+==========+=======+
|            1 |     1001 |        1 | A     |
|            2 |     1002 |        1 | B     |
|            3 |     1003 |        1 | A     |
|            4 |     3001 |        3 | A     |
|            5 |     3001 |        2 | B     |
|            6 |     4001 |        4 | A     |
|            7 |     4002 |        4 | A     |
+--------------+----------+----------+-------+

select * from person;
+----------+----------+-----------+------------+----------------+
| personid | lastname | firstname | hiredate   | enrollmentdate |
+==========+==========+===========+============+================+
|        1 | Goudar   | Anil      | 2016-08-16 | 2016-08-17     |
|        2 | Goudar   | Sunil     | 2018-09-16 | 2018-09-27     |
|        3 | Dambal   | Abhi      | 2018-05-07 | 2018-06-17     |
|        4 | Desai    | Arun      | 2018-05-07 | 2018-06-17     |
|        5 | Xam      | Sam       | 2018-12-08 | 2018-12-08     |
|        6 | Chatpati | Mangal    | 2018-10-10 | 2018-10-08     |
|        9 | Shankar  | Dev       | 2018-10-10 | 2018-10-08     |
|       10 | Shankar  | Mahadev   | 2018-08-10 | 2018-08-11     |
+----------+----------+-----------+------------+----------------+

现在我正在尝试获取部门详细信息以及属于该部门的学生人数。

这是我的查询，

select d.departmentid, d.name, sg.personid from department d inner join course c on c.departmentid = d.departmentid inner join student_grade sg on sg.courseid = c.courseid;
+--------------+--------------+----------+
| departmentid | name         | personid |
+==============+==============+==========+
|          101 | Computer Sci |        1 |
|          101 | Computer Sci |        1 |
|          101 | Computer Sci |        1 |
|          104 | Mech         |        3 |
|          104 | Mech         |        2 |
|          103 | EEE          |        4 |
|          103 | EEE          |        4 |
+--------------+--------------+----------+

但我想像 group by 子句一样获取每个部门的计数(distinct(personid))。但是我收到以下查询的错误。

select d.departmentid, d.name, count(distinct(sg.personid)) from department d inner join course c on c.departmentid = d.departmentid inner join student_grade sg on sg.courseid = c.courseid;
Cannot use non GROUP BY column 'departmentid' in query results without an aggregate function

请帮助我，我哪里错了。

Answer 1

使用这个：-

select d.departmentid, d.name, count(distinct(sg.personid)) 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;

Answer 2

你必须像使用聚合函数一样使用 group by

   select d.departmentid, d.name, 
count(distinct(sg.personid)) 
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

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