sql-join.md

   1 ---
   2 title: SQL joins
   3 layout: 2017/sheet
   4 category: Databases
   5 updated: 2018-12-06
   6 weight: -1
   7 ---
   8
   9 ### Example
  10
  11 ```
  12 SELECT * FROM order_items \
  13   LEFT OUTER JOIN orders \
  14   ON order_items.order_id = orders.id
  15 ```
  16 {: .-wrap}
  17
  18 Joins are typically added to `SELECT` statements to add more columns and records.
  19
  20 ### Diagram
  21
  22 ```
  23 SELECT * FROM `A` INNER JOIN `B`
  24 ```
  25 {: .-setup}
  26
  27 ```
  28 ┌────────┐
  29 │ A  ┌───┼────┐
  30 │    │ ∩ │    │
  31 └────┼───┘  B │
  32      └────────┘
  33 ```
  34 {: .-box-chars.-setup}
  35
  36 | Join             | What            |
  37 | ----             | ----            |
  38 | Inner join       | `∩`             |
  39 | Left outer join  | `A` + `∩`       |
  40 | Right outer join | `∩` + `B`       |
  41 | Full outer join  | `A` + `∩` + `B` |