leetcode_gym/_problems/sql_customers_who_never_order.md

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layout	title	last_solved	category	leetcode_url	status
page	customers who never order		sql	https://leetcode.com/problems/customers-who-never-order/solution/	solved

Problem

Suppose that a website contains two tables, the Customers table and the Orders table. Write a SQL query to find all customers who never order anything.

Table: Customers.

+----+-------+
| Id | Name  |
+----+-------+
| 1  | Joe   |
| 2  | Henry |
| 3  | Sam   |
| 4  | Max   |
+----+-------+
Table: Orders.

+----+------------+
| Id | CustomerId |
+----+------------+
| 1  | 3          |
| 2  | 1          |
+----+------------+
Using the above tables as example, return the following:

+-----------+
| Customers |
+-----------+
| Henry     |
| Max       |
+-----------+

Solution

method 1: not in - subquery

{% highlight sql %}

select name as 'Customers' from Customers where id not in ( select customerid from orders )

{% endhighlight %}

---

left join and filter nulls

{% highlight sql %}

select c.name as 'Customers' from Customers c left join Orders o on c.id=o.customerid where o.customerid is null

{% endhighlight %}