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141 lines
4.7 KiB
141 lines
4.7 KiB
# SQL Practise II
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```
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- one-to-many relationship between two models
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- using foreign keys
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- table joins: joining tables at foreign keys
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```
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##Objectives
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- Practice setting up a one-to-many relationship using foreign keys
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- Practise returning tables joined by foreign key
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##Schema - one-to-many
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We are going to relate two models: `users` and `regions`, for a potential meetup app
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- Create a database, call it `meetup_app`
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- Create a `users` table. Make it either in a `schema.sql` file or straight into the psql command line. Each `user` should have:
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- id
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- name
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- region_id
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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:
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```
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*many* users belong to *one* region.
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```
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```
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*one* region has *many* users.
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```
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</br>
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NOTE: There are a few basic data relationships used extensively in Rails applications, such as **one-to-many** and also **many-to-many**.
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Here is a good overview of SQL relationships including joins:
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http://code.tutsplus.com/articles/sql-for-beginners-part-3-database-relationships--net-8561
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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`.
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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.
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</br>
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```
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CONCEPTUAL QUESTION:
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Why is it that the foreign key lives in the many? Would it make sense to put the foreign key in the one?
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```
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- Create a `regions` table. Each region should have
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- A unique ID
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- A name
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- A radius in miles
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```
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CONCEPTUAL QUESTION:
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**NOTE:** the `regions` table has nothing to associate any users with it. Why do we not need anything here to relate or 'store' users?
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```
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If you put your schemas into a `schema.sql` file, run it with `psql -d meetup_app < schema.sql`.
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## Seed
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- 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`
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Test that your regions have been seeded.
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- 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;`
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Test that your users have been seeded.
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## Join Queries
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- Select all the regions and join to the regions all associated users. `SELECT * FROM regions . . . `
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EXAMPLE RESULT:
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```
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id | name | radius | id | name | region_id
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----+----------------+--------+----+-----------+-----------
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1 | Alpha Centauri | 8000 | 1 | Mork | 1
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1 | Alpha Centauri | 8000 | 2 | Mindy | 1
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2 | Procyon | 20000 | 3 | AlphaGo | 2
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2 | Procyon | 20000 | 4 | Lee Sedol | 2
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2 | Procyon | 20000 | 5 | Alvin | 2
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3 | Black Hole | 600 | 6 | Theodore | 3
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3 | Black Hole | 600 | 7 | Simon | 3
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(7 rows)
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```
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- 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.
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EXAMPLE RESULT:
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```
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id | name | radius | name
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----+----------------+--------+-----------
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1 | Alpha Centauri | 8000 | Mork
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1 | Alpha Centauri | 8000 | Mindy
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2 | Procyon | 20000 | AlphaGo
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2 | Procyon | 20000 | Lee Sedol
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2 | Procyon | 20000 | Alvin
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3 | Black Hole | 600 | Theodore
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3 | Black Hole | 600 | Simon
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(7 rows)
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```
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- 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`.
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EXAMPLE RESULT:
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```
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id | name | radius | user
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----+----------------+--------+-----------
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1 | Alpha Centauri | 8000 | Mork
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1 | Alpha Centauri | 8000 | Mindy
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2 | Procyon | 20000 | AlphaGo
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2 | Procyon | 20000 | Lee Sedol
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2 | Procyon | 20000 | Alvin
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3 | Black Hole | 600 | Theodore
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3 | Black Hole | 600 | Simon
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(7 rows)
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```
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## BONUS
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- Find all regions that have more than 1 user
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- Find all regions with a radius of more than 1000 and less than 3 users
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</br>
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</br>
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