我被困在一个 sql 查询上答案

【问题标题】：I am stuck on a sql query我被困在一个 sql 查询上
【发布时间】：2018-08-09 04:24:05
【问题描述】：

编写查询以列出不同的办公室（他们的地址）、该办公室的员工人数以及该办公室的员工作为销售代表的订单数量。

到目前为止，这是我在 mysql 中提出的：

SELECT Offices.addressLine1,
        Offices.addressLine2, 
        COUNT(Employees.employeeNumber),
        COUNT(Orders.orderNumber)
   FROM Offices,
        Employees,
        Customers,
        Orders
   WHERE Offices.officeCode=Employees.officeCode AND
          Employees.employeeNumber=Customers.salesRepEmployeeNumber AND
          Customers.customerNumber=Orders.customerNumber
   GROUP BY addressLine1;

http://www.richardtwatson.com/dm6e/images/general/ClassicModels.png

这也是我用作参考的数据模型的链接。我知道我的代码并不完全正确，但我不知道如何以一种有意义的方式将所有表连接在一起，所以任何帮助都将不胜感激，谢谢！

【问题讨论】：

这个查询得到了什么输出。并尝试group by Offices.addressLine1, Offices.addressLine2;
见meta.stackoverflow.com/questions/333952/…
您通常按您选择的列进行分组，除了那些参数设置函数的列！

标签： mysql sql select join

【解决方案1】：

您可以如下实现：

SELECT o.officecode
    ,o.addressLine1
    ,o.addressLine2
    ,count(e.employeeNumber)
    ,SUM(CASE 
            WHEN o.orderNumber IS NOT NULL
                THEN 1
            ELSE 0
            END) AS TotalOrders
FROM Offices AS o
INNER JOIN Employees AS e ON o.officecode = e.officecode
LEFT JOIN Customers AS c ON c.salesRepEmployeeNumber = e.employeeNumber
INNER JOIN Orders AS o ON o.customerNumber = c.customerNumber
GROUP BY o.officecode
    ,o.addressLine1
    ,o.addressLine2;

【讨论】：

【解决方案2】：

你可以试试下面的

SELECT 
Offices.officeCode,
Offices.addressLine1,
Offices.addressLine2,
COUNT(ISNULL(Employees.employeeNumber,0)) AS EmpCount,
COUNT(ISNULL(Orders.OrderNumber,0)) AS NoOfOrders    
FROM
Offices
INNER JOIN Employees
ON Offices.officeCode = Employees.officeCode
LEFT JOIN Customers
ON Employees.employeeNumber = Customers.salesRepEmployeeNumber
LEFT JOIN Orders
ON Customers.customerNumber = Orders.customerNumber
GROUP BY 
Offices.officeCode,
Offices.addressLine1,
Offices.addressLine2

【讨论】：

【解决方案3】：

跨多个表进行聚合时，您可能会得到不正确的结果。在这里，您需要计算distinct 员工，否则该计数将被订单数量夸大。请注意，count 函数会忽略 NULL，因此无需补偿 NULL，但您确实需要对 customere/orders 进行左连接以确保计算所有员工。

SELECT
      o.officecode
    , o.addressLine1
    , o.addressLine2
    , count(distinct e.employeeNumber)
    , count(o.orderNumber) AS TotalOrders
FROM Offices AS o
INNER JOIN Employees AS e ON o.officecode = e.officecode
LEFT JOIN Customers AS c ON c.salesRepEmployeeNumber = e.employeeNumber
LEFT JOIN Orders AS o ON o.customerNumber = c.customerNumber
GROUP BY
      o.officecode
    , o.addressLine1
    , o.addressLine2

【讨论】：