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|
+-----------------------+-------------------------------+

我想为每个number of employees

计算group by employee_hierarchy_id 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 consition和GROUP BY子句

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

如果您想要在没有员工的情况下仍显示办公记录，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

Answer 2

我找到了另一种方法，尝试这个子查询

SELECT SUM(number_of_employees) as 'number of employee per factory',factory_id
FROM (
SELECT COUNT(*) as 'number_of_employees',employee.factory_id
FROM factory,employee
WHERE factory.factory_id = employee.factory_id
GROUP BY employee.employee_hierarchy_id
) AS tmptbl 
GROUP BY tmptbl.factory_id

希望这有帮助！

SQL，COUNT在我的mysql查询中不起作用

2 个答案: