在表 orders 中找到订单数最多客户对应的 customer_number 。 数据保证订单数最多的顾客恰好只有一位。 表 orders 定义如下：

| order_number | customer_number | order_date | required_date | shipped_date | status | comment |
|--------------|-----------------|------------|---------------|--------------|--------|---------|
| 1            | 1               | 2017-04-09 | 2017-04-13    | 2017-04-12   | Closed |         |
| 2            | 2               | 2017-04-15 | 2017-04-20    | 2017-04-18   | Closed |         |
| 3            | 3               | 2017-04-16 | 2017-04-25    | 2017-04-20   | Closed |         |
| 4            | 3               | 2017-04-18 | 2017-04-28    | 2017-04-25   | Closed |         |

解题思路

此题考查两个知识点，一个是group，一个是limit。使用group针对customer_number做分组统计出订单数最多的用户。limit直接返回第一个，即为数量最多的用户。

解题答案

select customer_number from orders group by customer_number order by count(customer_number) desc limit 0,1;

进阶

如果存在多个用户订单数一样多，如何返回对应所有的customer_number.

SELECT
    customer_number
FROM
    orders o1
GROUP BY o1.customer_number
HAVING COUNT(*) = (
            SELECT
                COUNT(*)
            FROM
                orders o2
            GROUP BY o2. customer_number
            ORDER BY COUNT(*) DESC
            LIMIT 1
    );