183_Customers Who Never Order
Suppose that a website contains two tables, theCustomers
table and theOrders
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 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);
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