UofT-DSI | SQL - Assignment 2

[hayun120]

What changes are you trying to make? (e.g. Adding or removing code, refactoring existing code, adding reports)

I have completed Assignment 2, which consists of a conceptual map (png file), brief responses to some prompts (.md file), SQL queries (sql file), and ethics reflection (.md file).

What did you learn from the changes you have made?

I learned how to use COALESCE to handle NULL values, various Window functions linke DENSE_RANK() and ROW_NUMBER paired with PARTITION BY, as well as how to create temporary tables and using functions to modify them. I also learned the difference between Type 1 and Type 2 slowly changing dimensions.

Was there another approach you were thinking about making? If so, what approach(es) were you thinking of?

For Query 2 (numbering customer visits), either ROW_NUMBER() or DENSE_RANK() could work. I chose the latter because it handles ties more seamlessly. If a customer made multiple purchases on the same market date, they still get the same visit number rather than arbitrary sequential numbers.

Were there any challenges? If so, what issue(s) did you face? How did you overcome it?

Oh gosh! The last query was for sure a challenge. I kept running into problems with the final output still returning 'null' values for the newly created column. After multiple modifications, I realized it was because I initially placed COALESCE inside the subquery around the column, which did not catch the case where a product had no rows in vendor_inventory at all, so the entire subquery returned nothing rather than NULL. Moving COALESCE to wrap the outer subquery resolved this!

How were these changes tested?

Queries were run in DB Browser for SQL. Output tables were visually inspected after each query to confirm correct results.

A reference to a related issue in your repository (if applicable)

N/A

Checklist

• I can confirm that my changes are working as intended

[Dmytro-Bonislavskyi]

Well done!