mysql - 使用 count 来计算值和空值

Question

我有下面的查询

List the number of students graduated in 2009 by department. The results should show 0 for the departments that do not have any student graduated in 2009.

我在回答问题的第二部分时遇到了问题。截至目前，我的查询仅显示有学生毕业的部门。我不知道如何让表格显示没有任何学生毕业的部门。

我的查询看起来像这样

select d.name, count(s.major_id) as students from departments d
right join students s on s.major_id = d.id where extract( year from s.graduation_date ) = 2009
group by d.name

我的 table 是这样的

name    students
Math       2
Drama      1

如何让它显示其他部门没有学生毕业？

数据库是

create table departments (
    id      integer primary key,
    name    varchar(255)
);

insert into departments (id, name) values (10, 'Computer Science');
insert into departments (id, name) values (20, 'Math');
insert into departments (id, name) values (30, 'Drama');

create table faculty (
    id              integer primary key,
    name            varchar(255),
    department_id   integer references departments(id)
);

insert into faculty (id, name, department_id) values (1, 'Turing', 10);
insert into faculty (id, name, department_id) values (2, 'Newton', 20);
insert into faculty (id, name, department_id) values (3, 'Einstein', 20);
insert into faculty (id, name, department_id) values (4, 'Brando', 30);
insert into faculty (id, name, department_id) values (5, 'Joe', 30);
insert into faculty (id, name, department_id) values (6, 'Gray', 10);

create table students (
    id              integer primary key,
    name            varchar(255),
    graduation_date date,
    major_id        integer references departments(id)
);

insert into students (id, name, graduation_date, major_id) values
    (1, 'Joe', null, 10);
insert into students (id, name, graduation_date, major_id) values
    (2, 'Amy', '2009-04-22', 20);
insert into students (id, name, graduation_date, major_id) values
    (3, 'Max', null, 10);
insert into students (id, name, graduation_date, major_id) values
    (4, 'Sue', '2009-01-10', 20);
insert into students (id, name, graduation_date, major_id) values
    (5, 'Bob', '2009-03-05', 30);
insert into students (id, name, graduation_date, major_id) values
    (6, 'Kim', null, 20);
insert into students (id, name, graduation_date, major_id) values
    (7, 'Art', null, 30);
insert into students (id, name, graduation_date, major_id) values
    (8, 'Pat', '2005-07-11', 20);
insert into students (id, name, graduation_date, major_id) values
    (9, 'Lee', null, 10);

Answer 1

尝试使用 LEFT JOIN 而不是 RIGHT JOIN:

select d.name, count(s.major_id) as students 
from departments d
left join students s on s.major_id = d.id and 
                        extract( year from s.graduation_date ) = 2009
group by d.name

注意 extract( year from s.graduation_date ) = 2009 谓词应该放在 ON 子句中，否则 LEFT JOIN 变成一个内部连接。

输出:

 name            | students
=================+============
Computer Science |  0
Drama            |  1
Math             |  2