SQL Join

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