If I have three simple tables:

mysql> select * from customer;
+----+--------+
| ID | NAME |
+----+--------+
| 1 | Joey |
| 2 | Mike |
| 3 | Kellie |
+----+--------+
3 rows in set (0.00 sec)

mysql> select * from fruit;
+----+---------+
| ID | NAME |
+----+---------+
| 1 | Apples |
| 2 | Grapes |
| 3 | Oranges |
| 4 | Kiwis |
+----+---------+
4 rows in set (0.00 sec)

mysql> select * from purchases;
+----+---------+----------+
| ID | CUST_ID | FRUIT_ID |
+----+---------+----------+----
| 2 | 3 | 2 |
| 3 | 1 | 4 |
| 4 | 1 | 2 |
| 5 | 2 | 1 |
+----+---------+----------+----

I am having trouble understanding a relational query. How can I select
those fruits that Joey has not purchased?