sql-join: add

sql-join.md

@@ -0,0 +1,19 @@
+---
+title: SQL Join
+---
+
+```
+SELECT * FROM order_items LEFT OUTER JOIN orders ON order_items.order_id = orders.id
+```
+
+```
+   __  __
+ /   /\   \
+| A |u |B  |
+ \___\/__ /
+```
+
+* inner join = `u`
+* left outer join = `A` + `u`
+* right outer join = `u` + `B`
+* full outer join = `A` + `u` + `B`