Example
SELECT * FROM order_items \
LEFT OUTER JOIN orders \
ON order_items.order_id = orders.id
Joins are typically added to SELECT statements to add more columns and records.
Diagram
SELECT * FROM `A` INNER JOIN `B`
┌────────┐
│ A ┌───┼────┐
│ │ ∩ │ │
└────┼───┘ B │
└────────┘
| Join | What |
|---|---|
| Inner join | ∩ |
| Left outer join | A + ∩ |
| Right outer join | ∩ + B |
| Full outer join | A + ∩ + B |