Add blog post on SQL joins.

posts/sql-joins.md

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+---
+title: "SQL Joins"
+tags:
+  - sql
+  - joins
+  - database
+date: "2019-10-27T05:44:08.920Z"
+layout: post
+---
+#### Author: [Kimberly Collins](https://github.com/kacollins)
+
+## Database Normalization
+* Separate data into different tables
+* Usually represent each entity with an ID
+* Optimize for data storage efficienty, not human readability
+
+## Purposes of Joins
+* Use data from multiple tables
+* "Un-normalize" data
+  * Make data human readable
+  * View information about the entity, not just the ID
+
+## How to Get Started
+* Install SQL Server Management Studio or another query tool (maybe LinqPad!)
+* Install AdventureWorks sample database
+
+## Types of Joins
+
+### [CROSS JOIN](https://clips.twitch.tv/HelpfulRelatedMarrowMikeHogu)
+* Usage
+  * Returns all possible combinations of rows between two tables
+  * If one table has 2 rows and the other has 3 rows, cross joining them returns 6 rows
+  * Used for generating test data and for reports
+  * Otherwise, use it as a last resort
+* Example
+  * Department table has 16 rows
+  * Shift table has 3 rows
+  * CROSS JOIN will return the 48 possible combinations
+
+### [INNER JOIN](https://clips.twitch.tv/UnsightlyRichDogeDuDudu)
+* [Usage](https://clips.twitch.tv/AmusedGracefulLampRickroll)
+  * Returns rows that match on criteria
+  * Can (unintentionally) limit results or introduce duplicates
+  * "INNER" is optional but recommended
+  * Generally joining tables on one or more columns that exist in both
+    * Usually the primary key in one table and a foreign key in the other
+    * Other criteria can be used
+* Example
+  * ProductSubcategory table has a column for ProductCategoryID
+  * We want to see the name of the product category for each subcategory, not the ID
+
+### [LEFT OUTER JOIN](https://clips.twitch.tv/PlumpGoldenDaikonStrawBeary)
+* Usage
+  * Returns rows in the first table even if they don't have a match in the second table
+  * Can unintentionally introduce duplicates
+  * "OUTER" is optional
+  * Often used with a WHERE clause to only get the rows in the first table that don't have a match in the second table
+* Examples
+  * Show the subcategory for each product, if it has one
+  * Get the products that have never been sold (with WHERE clause)
+
+### RIGHT OUTER JOIN
+* Analogous to LEFT OUTER JOIN
+* Rarely used
+
+### FULL OUTER JOIN
+* Usage
+  * Returns rows from both tables even if they don't have matches in the other
+  * "OUTER" is optional
+  * Often used for comparison reports
+* Example
+  * Show salespeople assigned to each territory
+  * Include salespeople not assigned to any territory
+  * Include territories without any salespeople assigned
+
+## Learning More
+  * [Slides](https://docs.google.com/presentation/d/1JzqXzFljkzZ5gTVrgzIV7Hhgv70Ffpi2unwnNWxbyjE/edit)
+  * [Video](https://www.youtube.com/watch?v=2IGQFucnGR4)
+  * [Examples](github.com/kacollins/intro-to-sql-joins)
+  * OKC SQL meetup group - meets on the third Monday evening of the month
+  * Techlahoma's Slack
+    * Sign up at slack.techlahoma.org
+    * #databases and #okcsql channels
+    * @kimberlycollins
+
+-- [Kimberly](https://github.com/kacollins)