Leetcode: Customer Placing the Largest Number of Orders

Customer Placing the Largest Number of Orders



Similar Problems:


Query the customer_number from the orders table for the customer who has placed the largest number of orders.

It is guaranteed that exactly one customer will have placed more orders than any other customer.

The orders table is defined as follows:

| Column            | Type      |
|-------------------|-----------|
| order_number (PK) | int       |
| customer_number   | int       |
| order_date        | date      |
| required_date     | date      |
| shipped_date      | date      |
| status            | char(15)  |
| comment           | char(200) |

Sample Input

| 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 |         |

Sample Output

| customer_number |
|-----------------|
| 3               |

Explanation

The customer with number '3' has two orders, which is greater than either customer '1' or '2' because each of them  only has one order. 
So the result is customer_number '3'.

Follow up: What if more than one customer have the largest number of orders, can you find all the customer_number in this case?

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.


## Blog link: https://code.dennyzhang.com/customer-placing-the-largest-number-of-orders

# assume: only one match
select customer_number from orders
group by customer_number
order by count(1) desc
limit 1

## assum: multiple matches
##  1 1
##  2 1
##  3 1
##
##  1 1 1 1
##  1 1 2 1
##  1 1 3 1
##
##  select t1.customer_number
##  from (select customer_number, count(1) as count
##        from orders group by customer_number) as t1,
##        (select customer_number, count(1) as count
##        from orders group by customer_number) as t2
##  group by t1.customer_number
##  having max(t1.count) = max(t2.count)
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