183_Customers Who Never Order

Suppose that a website contains two tables, theCustomerstable and theOrderstable. 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 1 : OUTER JOIN

SELECT Name AS Customers
FROM Customers AS c LEFT OUTER JOIN Orders AS o
ON c.Id = o.CustomerId
GROUP BY c.Id        -- don't group by Name because Name can be duplicate
HAVING count(CustomerId) = 0;

Solution 2: Subqueries

SELECT Name AS Customers
FROM Customers
WHERE Id NOT IN (SELECT DISTINCT CustomerId
                FROM Orders);