Customers Who Never Order
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 |
+-----------+
執行左外連接,然後判斷customerid is null的記錄
select name,customerid from Customers c left outer join Orders o on c.id=o.customerid
+-------+------------+
| name | customerid |
+-------+------------+
| Joe | 1 |
| Henry | NULL |
| Sam | 3 |
| Max | NULL |
+-------+------------+
select name as Customers from (select name,customerid from Customers c left outer join Orders o on c.id=o.customerid) t where t.customerid is null;