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

Question

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

但是以下会导致错误。目的是仅在结果行集中显示那些下达 3 个以上订单的客户。我究竟做错了什么？

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错误是：错误代码：1111。组函数的使用无效

Answer 1

聚合函数（COUNT(), AVG(), SUM(),等）不能出现在WHERE子句中，因为它们是计算的。相反，它们属于一个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

Answer 2

一种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

Answer 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 子句中使用了 count 函数，它根据 count 计算返回 FALSE 字符串的 TRUE。

希望能帮助到你

Answer 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

Answer 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;