SQL,COUNT在我的mysql查询中不起作用

Question

我正在使用mysql 5.1.

这是我的数据库架构

office
+--------------+----------------+
| office_id    | office_name    |
+--------------+----------------+

employee
+--------------+-----------------------+------------+
| employee_id  | employee_hierarchy_id | office_id  |
+--------------+-----------------------+------------+

employee_hierarchy
+-----------------------+-------------------------------+
| employee_hierarchy_id | employee_hierarchy_description|
+-----------------------+-------------------------------+

我想要计算number of employees group by employee_hierarchy_id每一个office_id

这是我列出factory_id和employee_hierarchy_id的请求:

SELECT office_id, employee_hierarchy_id
FROM office, employee_hierarchy
ORDER BY office_id ASC 

如果我尝试添加计数,它只显示一行:

SELECT office_id, employee_hierarchy_id, COUNT(*)
FROM office, employee_hierarchy
ORDER BY office_id ASC

Answer 1

添加WHERE意见和GROUP BY条款

SELECT office_id, employee_hierarchy_id, COUNT(*)
FROM office, employee_hierarchy
WHERE office.factory_id = employee_hierarchy.factory_id
GROUP BY office_id, employee_hierarchy_id
ORDER BY office_id ASC

或推荐的语法

SELECT office_id, employee_hierarchy_id, COUNT(*)
FROM office INNER JOIN employee_hierarchy
        ON office.factory_id = employee_hierarchy.factory_id
GROUP BY office_id, employee_hierarchy_id
ORDER BY office_id ASC

更新1

如果你想在没有员工的情况下仍然显示办公室记录,请使用LEFT JOIN,否则INNER JOIN

SELECT  a.office_name , c.employee_hierarchy_description,
        COUNT(b.office_ID) totalCount
FROM    office a
        LEFT JOIN employee b
            ON a.office_ID = b.office_ID
        LEFT JOIN employee_hierarchy c
            ON b.employee_hierarchy_id = c.employee_hierarchy_id
GROUP BY a.office_name , c.employee_hierarchy_description
ORDER BY totalCount DESC