mysql - 查询统计每个经理的员工数量(经理的计数也为零)

Question

经理表:

create table Manager(id integer, name varchar(100));
insert into Manager(id, name) values(1, "M1");
insert into Manager(id, name) values(2, "M2");
insert into Manager(id, name) values(3, "M3");

员工表以及经理 ID(m_id)

create table Employee(id integer, m_id integer, name varchar(100));
insert into Employee(id, m_id, name) values(11, 1,"E1");
insert into Employee(id, m_id, name) values(12, 1,"E2");
insert into Employee(id, m_id, name) values(13, 2,"E3");

执行的查询:

select M.id, M.name, count(E.id) from Manager as M, Employee as E where E.m_id=M.id group by E.m_id, M.id, M.name;

输出:

1   M1  2
2   M2  1 

预期输出:

1   M1  2
2   M2  1
3   M3  0

请帮助我如何将 M3 经理以及分配给他的 0 员工包括在内？

Answer 1

您应该使用左连接

SELECT M.id, M.name, count(E.id) from Manager as M LEFT JOIN Employee as E ON E.m_id=M.id GROUP BY E.m_id, M.id, M.name;