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A coding challenge about databases
PLpgSQL JavaScript
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Author: @shiryz

Maintainer: @shiryz


Databases morning challenge

The purpose of this challenge is to connect to a remote database, construct complicated queries, learn about joins and use some subqueries.

Basic set-up

  • Fork this repo
  • Clone your forked version.
  • cd into the repo in your command line $ cd db-morning-challenge
  • Install dependencies with $ npm install

Connect to a remote database:

Using a local database is ok, but only you have access to it, as it is on your PC.

To share a database between multiple developers, we can use remote databases - databases that are hosted on the cloud. We then connect to them the same way we connect to our local ones, just using a different URL - each database will have its own unique url.

Heroku has a service for hosting SQL databases - once you've create a heroku app (as you have been doing since week 4), you can then use a heroku add-on to add a database, either using the heroku command line interface (CLI) or on the website:

NOTE: We are just creating a heroku app so that we can use the heroku remote database - our code doesn't actually make a usable website

These should happen after you have cloned and ran npm i

Pick One:

Using the CLI

  • Create an app on Heroku heroku create app-name-here --region eu (If you haven't yet installed Heroku CLI see here
  • Push to Heroku git push heroku master
  • Create a new database on Heroku: heroku addons:create heroku-postgresql:hobby-dev --as USERS_DB
  • Find the database url, either on the heroku dashboard for your project, under settings (click reveal config vars), or by using heroku config -s | grep USERS_DB (note: make sure to remove the quotations around the url when using this method).
  • Back in your command line, create a config.env file with the url of your new database. You can do that like this  $ echo "export DATABASE_URL = {YOUR_COPIED_DATABASE_URL}" >> "config.env"
  • Build your database by running: $ node database/db_build.js

Using the website

  • Log into the heroku website
  • On the heroku dashboard, create a new app, giving it a unique name
  • Go to the Resources table and search postgres in the Add-ons - select the free version of Heroku Postgres and click Provision
  • Go to Settings->Reveal Config Variables to get the url of your database
  • Back in your command line, create a config.env file with the url of your new database. You can do that like this:  $ echo "export DATABASE_URL = {YOUR_COPIED_DATABASE_URL}" >> "config.env"
  • Build your database by running: $ node database/db_build.js

Using Heroku databases

You and your partner now both have access to the same database from the command line with psql {YOUR_COPIED_DATABASE_URL}

For each of the challenges below, write and test your queries in the command line and save them in a text editor when you're happy with them, so you can refer back to them later.

Challenge 1

One of the tables you have created is a books table that looks like this:

book_id book_name year max_reservation_time library

your challenge is to construct a query that returns the following columns:

  • book_id,
  • book_name,
  • max_reservation_time

AND to return only the books that can be reserved for a time greater than the average reservation time of its own library group. (hint: We're not trying to find the overall average across all libraries)

Hint: try using sub queries

You should expect to see this:

book_id book_name max_reservation_time
1 Javascript: The Good Parts 21
5 Pride and Prejudice 21

Challenge 2:

Your database contains 3 more tables:

Mentors For storing details of FAC mentors.

name location

Posts For things that have been posted by mentors:

id mentor_name

Likes For the likes posts have received, and the person who liked it

mentor_name post_id

Challenge Pt 1

  • Construct a query that returns the names of mentors and the number of likes each mentor got, in total, for all their posts.

    You should expect to see this:

mentor_name count
Shireen 9
Tom 4
Steve 4

Challenge Pt 2

  • Construct a query that returns the location and the post id, for each of the likes. The post_id is the id of the post that has been liked and the location is the location of the mentor that did the liking.

    You should expect to see this:

location post_id
Nazareth 20
Nazareth 44
Nazareth 19
Nazareth 57
Nazareth 32
Nazareth 20
Nazareth 19
Nazareth 44
Nazareth 20
Nazareth 19
London 19
London 57
London 32
London 44
London 32
London 44
London 20

Challenge 3 (bonus, you can try this one at home!)

Building on the queries you wrote in level 2, construct another query that returns the average number of likes per post in each location.

You should expect to see this:

location avg
London 4
Nazareth 3
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