Lightspeed

Problem to Solve

In 1880, Albert A. Michelson ran a set of experiments to approximately measure the speed of light (then unknown!). The experiments paved the way towards our understanding of light as we know it today.

In a database called lightspeed.db, within a table called observations, use SQL to analyze the data from Michelson’s experiments.

Distribution Code

For this problem, you’ll need to download lightspeed.db, along with a few .sql files in which you’ll write your queries.

Download the distribution code

Log into cs50.dev, click on your terminal window, and execute cd by itself. You should find that your terminal window’s prompt resembles the below:

Next execute

wget https://cdn.cs50.net/sql/2024/spring/ao/lightspeed.zip

in order to download a ZIP called lightspeed.zip into your codespace.

Then execute

unzip lightspeed.zip

to create a folder called lightspeed. You no longer need the ZIP file, so you can execute

rm lightspeed.zip

and respond with “y” followed by Enter at the prompt to remove the ZIP file you downloaded.

Now type

cd lightspeed

followed by Enter to move yourself into (i.e., open) that directory. Your prompt should now resemble the below.

lightspeed/ $

If all was successful, you should execute

ls

and see a database named lightspeed.db alongside a few .sql files. Executing sqlite3 lightspeed.db should open the database in sqlite3, via which you’ll execute SQL queries. If not, retrace your steps and see if you can determine where you went wrong!

Schema

Michelson’s data includes 100 trials across 5 separate experiments. For each trial, Michelson recorded the approximate speed of light in kilometers per second.

In lightspeed.db you’ll find a single table, observations. In the observations table, you’ll find the following columns:

id, which uniquely identifies each observation of the speed of light
experiment, which is the experiment number (1–5)
trial, which is the trial number within the experiment (1–20)
speed, which is the recorded speed of light in kilometers per second

Specification

`1.sql`

In 1.sql, write a query to return the average speed of light across all experiments.

Your query should return a table with a single column, called “average_speed”, and a single row.

`2.sql`

In 2.sql, write a query to return the average speed of light within each experiment.

Your query should return a table with two columns: one called “experiment”, which contains the experiment number, and one called “average_speed”, which is the average speed of light recorded in that experiment.

`3.sql`

In 3.sql, write a query to find the 5 most frequently recorded observations of speed across all experiments and the number of times that observation of speed was recorded.

Your query should return a table with two columns: one called “speed”, which is the speed of light recorded, and one called “count”, which is the number of times that observation appears across all experiments.

`4.sql`

In 4.sql, write a query to find the speed(s) of light found in common across all 5 experiments.

Your query should return a table with a single column, “speed”, representing the speed(s) of light common across all 5 experiments.

Usage

To test your queries as you write them in your .sql files, you can query the database by running

.read FILENAME

where FILENAME is the name of the file containing your SQL query. For example,

.read 1.sql

Assessment

This problem will be assessed as other query-based problems have been: along the axes of correctness, design, and style. As with past problems, your score is computed by summing the points you receive across each axis.

`check50`

check50 cs50/problems/2024/sql/lightspeed

How to Submit

After you submit, be sure to check your autograder results. If you see SUBMISSION ERROR: missing files (0.0/1.0), it means your file was not named exactly as prescribed (or you uploaded it to the wrong problem).

Correctness in submissions entails everything from reading the specification, writing code that is compliant with it, and submitting files with the correct name. If you see this error, you should resubmit right away, making sure your submission is fully compliant with the specification. The staff will not adjust your filenames for you after the fact!

Ensure your terminal’s working directory is the lightspeed folder. Your prompt should resemble the below:

lightspeed/ $

When you type ls to list the files in your working directory, you should see your .sql files for lightspeed.

Zip up your solution files by executing the following:

zip lightspeed-solutions.zip *.sql

Want to learn about this command?

Notice this command has three parts:

zip, which is the name of the tool which will create a .zip file
lightspeed-solutions.zip, which is the name of the .zip file to create
*.sql, which represents the file(s) to include.
- *.sql matches all files that end with .sql, as * is a “wildcard” character which matches any set of characters (similar to % in SQL!).

Next, download lightspeed-solutions.zip by control-clicking or right-clicking on the file in your codespace’s file browser and choosing Download.

Go to CSCI E-151’s Gradescope page.

Click Assessment Opportunity: Lightspeed.

Drag and drop your .zip file to the area that says Drag & Drop. Be sure that each .sql file is correctly named exactly as prescribed above, lest the autograder fail to run on your submission! Note that your submission is considered incomplete if any of the files are missing—be sure they’re all there!

Click Upload.

You should see a message that says “Assessment Opportunity: Lightspeed submitted successfully!”

Be sure to double-check your autograder results before moving on!

Acknowledgements

Data based on the morley data set from the datasets package in base R, stat.ethz.ch/R-manual/R-devel/library/datasets/html/morley.html.