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 # Schema
 
 | BUSINESSES               |              | COMPUTERS   |         | EVENTS      |              |
+| ------------------------ | ------------ | ----------- | ------- | ----------- | ------------ |
 | id                       | int          | id          | int     | id          | int          |
 | name                     | varchar(255) | active      | tinyint | computer_id | int          |
 | business_relationship_id | int          | business_id | int     | occurred_at | datetime     |
@@ -9,6 +10,91 @@
 
 # Lessons
 
+Links:
+* https://blog.codinghorror.com/a-visual-explanation-of-sql-joins/
+
+`WHERE` is applied before `GROUP BY`, `HAVING` is applied after and can filter on aggregates.
+
+
+## Inner join
+
+Produces a set of records that match in both Table A and Table B.
+
+```mysql
+SELECT * FROM table_a
+INNER JOIN table_b
+ON table_a.name = table_b.name;
+```
+
+```text
+  A |   | B
+ ===========
+(   | X |   )
+ ===========
+```
+
+
+## Full Outer Join
+
+Produces the set of results from both table A and table B, with matching records from both sides where available. 
+If there is no match the missing side will contain null.
+
+```mysql
+SELECT * FROM table_a
+FULL OUTER JOIN table_b
+ON table_a.name = table_b.name;
+```
+
+```text
+  A |   | B
+ ===========
+( X | X | X )
+ ===========
+```
+
+## LEFT OUTER JOIN
+
+Produces a complete set of records from table A, with the matching records (where available) in table B.
+If there is no match, the right side will contain a null.
+
+
+To produce a set of results found in table_a but not table_b.
+
+```mysql
+SELECT *
+FROM table_a
+LEFT OUTER JOIN table_b
+ON table_a.name table_b.name
+where table_b.id IS NULL;
+```
+
+```text
+  A |   | B
+ ===========
+( X |   |   )
+ ===========
+```
+
+To produce a set of results unique to table_a and table_b.
+
+```mysql
+SELECT *
+FROM table_a
+FULL OUTER JOIN table_b
+ON table_a.name = table_b.name
+WHERE table_a.id IS NULL
+OR table_b.id IS NULL;
+```
+
+```text
+  A |   | B
+ ===========
+( X |   | X )
+ ===========
+ ```
+
+# Questions
+
 1. Which businesses have more than N computers?
 2. Which computer had the most events in a single day?
 3. What are the top 10 most active computers (day, week, month)?  .i.e emits the most events.