SELECT * FROM order_items LEFT OUTER JOIN orders ON order_items.order_id = orders.id
.--------.
| A .---+----.
| | ∩ | |
'----+---' B |
'--------'
- inner join =
∩
- left outer join =
A
+∩
- right outer join =
∩
+B
- full outer join =
A
+∩
+B