Leetcode: Customers Who Never Order

Question

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 |
+———–+

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Solution

My Solution

# Write your MySQL query statement below
select Name
from Customers
where Id not in 
(select distinct c.Id
from Customers c, Orders o
where c.Id=o.CustomerId)

Other′s Solution

# Write your MySQL query statement below
select Name
from Customers a left join Orders b
on a.Id=b.CustomerId
where CustomerId is null

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Take-home Message

1. 
   
2. 
   

where CustomerId is null
null can be used on where