在连接中使用 Where 子句，以及 Group by 和 Order By

Posted 2023-05-09

【中文标题】在连接中使用 Where 子句，以及 Group by 和 Order By

【英文标题】：Using Where clause in a join, with Group by and Order By

【发布时间】：2012-05-12 02:16:20

【问题描述】：

我正在使用 mysql 经典模型数据库。以下查询工作正常（注意 where 子句）

select 
    customers.customerNumber as 'Customer ID',
    customers.customerName as 'Customer Name',
    count(orders.orderNumber) as 'Total Orders Placed'
from customers
left join orders on customers.customerNumber = orders.customerNumber
where customers.customerNumber > 200
group by
    customers.customerNumber
order by 
    3 asc

但以下导致错误。目的是仅显示结果行集中那些下达 3 个以上订单的客户。我做错了什么？

select 
    customers.customerNumber as 'Customer ID',
    customers.customerName as 'Customer Name',
    count(orders.orderNumber) as 'Total Orders Placed'
from customers
left join orders on customers.customerNumber = orders.customerNumber
where count(orders.orderNumber) > 3
group by
    customers.customerNumber
order by 
    3 asc

MySQL 错误为：错误代码：1111. 组函数使用无效

【参考方案1】：

聚合函数（COUNT(), AVG(), SUM(), 等）不能出现在 WHERE 子句中，因为它们的计算时间。相反，它们属于HAVING 子句：

select 
    customers.customerNumber as 'Customer ID',
    customers.customerName as 'Customer Name',
    count(orders.orderNumber) as 'Total Orders Placed'
from customers
left join orders on customers.customerNumber = orders.customerNumber
group by
    customers.customerNumber
HAVING count(orders.orderNumber) > 3
order by 
    3 asc

【讨论】：

而且由于我们希望客户有超过 3 个订单，因此将 left join 转为 inner join 没有任何害处

【参考方案2】：

更有效的方法是通过聚合将组移动到嵌套派生表中并加入该表。

SELECT 
  c.customerNumber AS 'Customer ID',
  c.customerName AS 'Customer Name',
  o.orderCount AS 'Total Orders Placed'
FROM 
  customers c
LEFT JOIN
  (SELECT
     customerNumber, 
     COUNT(*) AS orderCount
   FROM
     orders
   GROUP BY
     customerNumber) o
 ON
   c.customerNumber = o.customerNumber
 WHERE
   o.orderCount > 3
 ORDER BY
   3

【讨论】：

效率更高或更低（或者它是否无关紧要，优化器会在内部找出一个查询并将其转换为另一个）取决于优化器。

【参考方案3】：

在where子句中有一种间接使用聚合函数的方法

您可以将查询重写为

select customers.customerNumber as 'Customer ID',
       customers.customerName as 'Customer Name',
       count(orders.orderNumber) as 'Total Orders Placed'
  from customers left join orders on customers.customerNumber =
                                     orders.customerNumber
 where
 (SELECT CASE
        WHEN count(orders.orderNumber) > 3 THEN
             'TRUE'
        ELSE
             'FALSE'
        END
   FROM DUAL) = 'TRUE'
 group by customers.customerNumber
 order by 3 asc

这里在 select 子句中使用 count 函数，根据 count 计算返回 FALSE 字符串的 TRUE。

希望对你有帮助

【参考方案4】：

请试试这个

select 
  customers.customerNumber as 'Customer ID',
  customers.customerName as 'Customer Name',
  count(orders.orderNumber) as 'Total Orders Placed'
from customers
inner join orders on customers.customerNumber = orders.customerNumber
group by
  customers.customerNumber, 
  customers.customerName
having count(orders.orderNumber) > 3
order by 
  3 asc

【参考方案5】：

这是您要实现的目标的简化版本。

SELECT 
    customers.customerName,
    COUNT(orders.orderNumber) totalOrders
FROM
    customers
        JOIN
    orders ON customers.customerNumber = orders.customerNumber
GROUP BY customers.customerName
HAVING totalOrders > 2;