panpsychism/ruby_rails_sql_resources/sql_joins.md

SQL Practise II

- one-to-many relationship between two models
- using foreign keys 
- table joins: joining tables at foreign keys

##Objectives

• Practice setting up a one-to-many relationship using foreign keys
• Practise returning tables joined by foreign key

##Schema - one-to-many

We are going to relate two models: users and regions, for a potential meetup app

• Create a database, call it meetup_app
• Create a users table. Make it either in a schema.sql file or straight into the psql command line. Each user should have:
  • id
  • name
  • region_id

In the future, we would want our users to be able to subscribe to a regional group so they can see all the other users in their area. In this case, we want each user only to be associated with one region at a time. This is a one-to-many relationship, where:

*many* users belong to *one* region.

*one* region has *many* users.

NOTE: There are a few basic data relationships used extensively in Rails applications, such as **one-to-many** and also **many-to-many**.

Here is a good overview of SQL relationships including joins:
http://code.tutsplus.com/articles/sql-for-beginners-part-3-database-relationships--net-8561

In a one-to-many relationship, the foreign key lives in the many. In this case, users is the many, because many users belong to one region.

You might notice that the users schema has a region_id. This is the foreign key. It will be a number: the id of the region to which the user belongs.

CONCEPTUAL QUESTION:

Why is it that the foreign key lives in the many? Would it make sense to put the foreign key in the one?

• Create a regions table. Each region should have
  • A unique ID
  • A name
  • A radius in miles

CONCEPTUAL QUESTION:
    
**NOTE:** the `regions` table has nothing to associate any users with it. Why do we not need anything here to relate or 'store' users?

If you put your schemas into a schema.sql file, run it with psql -d meetup_app < schema.sql.

Seed

• In a regionseed.sql file, write the INSERTS for three different regions. Alternatively, write the INSERTS straight into the psql command line. If you made a file, run it with psql -d meetup_app < regionseed.sql

Test that your regions have been seeded.

• In a userseed.sql file, write the INSERTS for seven different users. Alternatively, write the INSERTS straight into the psql command line. The region_id will correspond to the id of one of the previously created regions. You check the region ids by just selecting the regions in psql: SELECT * FROM regions;

Test that your users have been seeded.

Join Queries

• Select all the regions and join to the regions all associated users. SELECT * FROM regions . . .

EXAMPLE RESULT:

 id |      name      | radius | id |   name    | region_id 
----+----------------+--------+----+-----------+-----------
  1 | Alpha Centauri |   8000 |  1 | Mork      |         1
  1 | Alpha Centauri |   8000 |  2 | Mindy     |         1
  2 | Procyon        |  20000 |  3 | AlphaGo   |         2
  2 | Procyon        |  20000 |  4 | Lee Sedol |         2
  2 | Procyon        |  20000 |  5 | Alvin     |         2
  3 | Black Hole     |    600 |  6 | Theodore  |         3
  3 | Black Hole     |    600 |  7 | Simon     |         3
(7 rows)

• Refine your result so that you don't get the user's id and region_id displayed. You just want the region id, region name, region radius, and user name.

EXAMPLE RESULT:

 id |      name      | radius |   name    
----+----------------+--------+-----------
  1 | Alpha Centauri |   8000 | Mork
  1 | Alpha Centauri |   8000 | Mindy
  2 | Procyon        |  20000 | AlphaGo
  2 | Procyon        |  20000 | Lee Sedol
  2 | Procyon        |  20000 | Alvin
  3 | Black Hole     |    600 | Theodore
  3 | Black Hole     |    600 | Simon
(7 rows)

• Now we have two columns called name. Refine the result further using an alias. Do the same query you did before, but this time, use AS to change the user's name column to user.

EXAMPLE RESULT:

 id |      name      | radius |   user    
----+----------------+--------+-----------
  1 | Alpha Centauri |   8000 | Mork
  1 | Alpha Centauri |   8000 | Mindy
  2 | Procyon        |  20000 | AlphaGo
  2 | Procyon        |  20000 | Lee Sedol
  2 | Procyon        |  20000 | Alvin
  3 | Black Hole     |    600 | Theodore
  3 | Black Hole     |    600 | Simon
(7 rows)

BONUS

• Find all regions that have more than 1 user
• Find all regions with a radius of more than 1000 and less than 3 users