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Assignment 2 Answers
samojoo Aug 18, 2025
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Added updated Relationship explanation and SQL Updates
Aug 24, 2025
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/* ASSIGNMENT 2 */
/* SECTION 2 */

-- COALESCE
/* 1. Our favourite manager wants a detailed long list of products, but is afraid of tables!
We tell them, no problem! We can produce a list with all of the appropriate details.

Using the following syntax you create our super cool and not at all needy manager a list:

SELECT
product_name || ', ' || product_size|| ' (' || product_qty_type || ')'
FROM product

But wait! The product table has some bad data (a few NULL values).
Find the NULLs and then using COALESCE, replace the NULL with a
blank for the first problem, and 'unit' for the second problem.

HINT: keep the syntax the same, but edited the correct components with the string.
The `||` values concatenate the columns into strings.
Edit the appropriate columns -- you're making two edits -- and the NULL rows will be fixed.
All the other rows will remain the same.) */
SELECT product_name, product_size, product_qty_type
FROM product
WHERE product_size ISNULL OR product_qty_type ISNULL;

SELECT
product_name || ', ' || coalesce(product_size, '')|| ' (' || coalesce (product_qty_type, 'unit') || ')' as Product_update
FROM product;


--Windowed Functions
/* 1. Write a query that selects from the customer_purchases table and numbers each customer’s
visits to the farmer’s market (labeling each market date with a different number).
Each customer’s first visit is labeled 1, second visit is labeled 2, etc.

You can either display all rows in the customer_purchases table, with the counter changing on
each new market date for each customer, or select only the unique market dates per customer
(without purchase details) and number those visits.
HINT: One of these approaches uses ROW_NUMBER() and one uses DENSE_RANK(). */

SELECT
customer_id,
market_date,
row_number() OVER (PARTITION by customer_id ORDER BY market_date) AS visit_count
FROM customer_purchases
ORDER by customer_id, market_date;



/* 2. Reverse the numbering of the query from a part so each customer’s most recent visit is labeled 1,
then write another query that uses this one as a subquery (or temp table) and filters the results to
only the customer’s most recent visit. */
SELECT
customer_id,
market_date,
row_number() OVER (PARTITION by customer_id ORDER BY date(market_date) DESC) AS visit_count
FROM customer_purchases
ORDER by customer_id, visit_count;

SELECT customer_id, market_date, visit_count
FROM (
SELECT
customer_id,
market_date,
row_number() OVER (PARTITION by customer_id ORDER BY date(market_date) DESC) AS visit_count
FROM customer_purchases )
WHERE visit_count = 1
ORDER by customer_id;
/* 3. Using a COUNT() window function, include a value along with each row of the
customer_purchases table that indicates how many different times that customer has purchased that product_id. */

SELECT customer_id, product_id, market_date, COUNT(*) OVER (PARTITION BY customer_id, product_id) AS total_customer_purchases
FROM customer_purchases;


-- String manipulations
/* 1. Some product names in the product table have descriptions like "Jar" or "Organic".
These are separated from the product name with a hyphen.
Create a column using SUBSTR (and a couple of other commands) that captures these, but is otherwise NULL.
Remove any trailing or leading whitespaces. Don't just use a case statement for each product!

| product_name | description |
|----------------------------|-------------|
| Habanero Peppers - Organic | Organic |

Hint: you might need to use INSTR(product_name,'-') to find the hyphens. INSTR will help split the column. */

SELECT
product_name,
CASE
WHEN instr(product_name, '-') > 0
THEN TRIM(substr(product_name, INSTR(product_name, '-') + 1))
ELSE NULL
END AS Product_desc
FROM product;


/* 2. Filter the query to show any product_size value that contain a number with REGEXP. */

SELECT *
FROM product
WHERE product_size REGEXP '[0-9]';

-- UNION
/* 1. Using a UNION, write a query that displays the market dates with the highest and lowest total sales.

HINT: There are a possibly a few ways to do this query, but if you're struggling, try the following:
1) Create a CTE/Temp Table to find sales values grouped dates;
2) Create another CTE/Temp table with a rank windowed function on the previous query to create
"best day" and "worst day";
3) Query the second temp table twice, once for the best day, once for the worst day,
with a UNION binding them. */

SELECT 'Best Day' as Day, s.date AS market_date, s.total_sales
FROM(SELECT date(market_date) AS date, SUM(quantity * cost_to_customer_per_qty) AS total_sales
FROM customer_purchases
GROUP BY date(market_date)) AS s
WHERE s.total_sales = (SELECT max(total_sales)
FROM (SELECT date(market_date) AS date, SUM(quantity * cost_to_customer_per_qty) AS total_sales
FROM customer_purchases
GROUP BY date(market_date)))
UNION
SELECT 'Worst Day' as Day, s2.date AS market_date, s2.total_sales
FROM(SELECT date(market_date) AS date, SUM(quantity * cost_to_customer_per_qty) AS total_sales
FROM customer_purchases
GROUP BY date(market_date)) AS s2
WHERE s2.total_sales = (SELECT min(total_sales)
FROM (SELECT date(market_date) AS date, SUM(quantity * cost_to_customer_per_qty) AS total_sales
FROM customer_purchases
GROUP BY date(market_date)));



/* SECTION 3 */

-- Cross Join
/*1. Suppose every vendor in the `vendor_inventory` table had 5 of each of their products to sell to **every**
customer on record. How much money would each vendor make per product?
Show this by vendor_name and product name, rather than using the IDs.

HINT: Be sure you select only relevant columns and rows.
Remember, CROSS JOIN will explode your table rows, so CROSS JOIN should likely be a subquery.
Think a bit about the row counts: how many distinct vendors, product names are there (x)?
How many customers are there (y).
Before your final group by you should have the product of those two queries (x*y). */


SELECT v.vendor_name, p.product_name, sum(coalesce(v2.original_price,0) * 5) AS revenue
FROM vendor_inventory v2
JOIN vendor v ON v.vendor_id = v2.vendor_id
JOIN product p ON p.product_id = v2.product_id
CROSS JOIN customer c
GROUP BY v.vendor_name, P.product_name
ORDER BY v.vendor_name, P.product_name, revenue DESC;

-- INSERT
/*1. Create a new table "product_units".
This table will contain only products where the `product_qty_type = 'unit'`.
It should use all of the columns from the product table, as well as a new column for the `CURRENT_TIMESTAMP`.
Name the timestamp column `snapshot_timestamp`. */
CREATE TABLE product_units AS
SELECT p.*, CURRENT_TIMESTAMP AS snapshot_timestamp
FROM product p
WHERE lower(trim(p.product_qty_type)) = 'unit';
SELECT *
FROM product_units;


/*2. Using `INSERT`, add a new row to the product_units table (with an updated timestamp).
This can be any product you desire (e.g. add another record for Apple Pie). */
INSERT INTO product_units
VALUES (10, 'Eggs', '1 dozen', 6, 'unit', CURRENT_TIMESTAMP);


-- DELETE
/* 1. Delete the older record for the whatever product you added.

HINT: If you don't specify a WHERE clause, you are going to have a bad time.*/
DELETE FROM product_units
WHERE product_name = 'Eggs'
AND snapshot_timestamp = ( SELECT MIN(snapshot_timestamp)
FROM product_units
WHERE product_name ='Eggs');


-- UPDATE
/* 1.We want to add the current_quantity to the product_units table.
First, add a new column, current_quantity to the table using the following syntax.

ALTER TABLE product_units
ADD current_quantity INT;



Then, using UPDATE, change the current_quantity equal to the last quantity value from the vendor_inventory details.

HINT: This one is pretty hard.
First, determine how to get the "last" quantity per product.
Second, coalesce null values to 0 (if you don't have null values, figure out how to rearrange your query so you do.)
Third, SET current_quantity = (...your select statement...), remembering that WHERE can only accommodate one column.
Finally, make sure you have a WHERE statement to update the right row,
you'll need to use product_units.product_id to refer to the correct row within the product_units table.
When you have all of these components, you can run the update statement. */

ALTER TABLE product_units
ADD current_quantity INT;


SELECT *
FROM product_units;

UPDATE product_units AS pu
SET current_quantity = COALESCE(( SELECT SUM(vi.quantity)
FROM vendor_inventory vi
WHERE vi.product_id = pu.product_id
AND date(vi.market_date) = (
SELECT MAX(date(vi2.market_date))
FROM vendor_inventory vi2
WHERE vi2.product_id = pu.product_id)),0);

SELECT *
FROM product_units;