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Case study solutions for #8weekstudychallenge. 🍽 danny's dinner.

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Case Study #1 - Danny's Diner

Problem Statement

Danny wants to use the data to answer a few simple questions about his customers, especially about their visiting patterns, how much money they’ve spent and also which menu items are their favourite. Having this deeper connection with his customers will help him deliver a better and more personalised experience for his loyal customers.

Entity Relationship Diagram

Danny's Diner

Case Study Questions

Each of the following case study questions can be answered using a single SQL statement:

  1. What is the total amount each customer spent at the restaurant?
  2. How many days has each customer visited the restaurant?
  3. What was the first item from the menu purchased by each customer?
  4. What is the most purchased item on the menu and how many times was it purchased by all customers?
  5. Which item was the most popular for each customer?
  6. Which item was purchased first by the customer after they became a member?
  7. Which item was purchased just before the customer became a member?
  8. What is the total items and amount spent for each member before they became a member?
  9. If each $1 spent equates to 10 points and sushi has a 2x points multiplier - how many points would each customer have?
  10. In the first week after a customer joins the program (including their join date) they earn 2x points on all items, not just sushi - how many points do customer A and B have at the end of January?

Solution

The complete syntax is here. Software used is Microsoft SQL Server.

1. What is the total amount each customer spent at the restaurant?

  • Use the aggregate function SUM to find the total amount spent by customers and use GROUP BY to aggregate.
  • Two tables are needed for this query so use JOIN to match tables(sales and menu).
Select s.customer_id, SUM(m.price) AS total_sales
From sales s
Join menu m
  ON s.product_id = m.product_id
Group by s.customer_id

Answer

customer_id total_sales
A 76
B 74
C 36

The results are:

  • Customer A spent $76
  • Customer B spent $74
  • Customer C spent $36

2. How many days has each customer visited the restaurant?

  • To get unique days use COUNT DISTINCT on the order_date column and GROUP BY customer_id.
Select customer_id, COUNT(DISTINCT order_date) AS days_visited
From sales
Group by customer_id

Answer

customer_id days_visited
A 4
B 6
C 2

The results are:

  • Customer A visited 4 times.
  • Customer B visited 6 times.
  • Customer C visited 2 times.

3. What was the first item from the menu purchased by each customer?

  • For this query create a CTE the WITH function. In the CTE, use DENSE_RANK and OVER(PARTITION BY ORDER BY) to create a new column which ranks the item based on order_date.
  • DENSE_RANK is used for this query as it assigns a rank to each row within a partition of a result set.
  • We do not know which item was ordered fist, hence I want to show the result as the same rank if they ordered separate items on the same date. So, we add a WHERE clause to see rank = 1 and group by customer_id and product_name.
WITH ranked_item as (
   Select s.customer_id, s.order_date ,m.product_name,
    DENSE_RANK() OVER (PARTITION BY s.customer_id ORDER BY  s.order_date) AS ranked
     From sales s
     Join menu m
        ON s.product_id = m.product_id
		)
Select customer_id, product_name
From ranked_item
Where ranked = '1'
Group by customer_id, product_name

Answer

customer_id product_name
A curry
A sushi
B curry
C ramen

The results are:

  • Customer A has curry and sushi as the first orders.
  • Customer B has curry as the first order.
  • Customer C has ramen as the first order.

4. What is the most purchased item on the menu and how many times was it purchased by all customers?

  • use COUNT to get the number of products and then order them in DESC order.
  • In the SELECT statement add TOP to get the highest number of purchased item.
Select TOP 1
m.product_name, COUNT(s.product_id) as count_product
From menu m
Join sales s
  ON m.product_id = s.product_id
Group by m.product_name
Order by count_product DESC

Answer

product_name count_product
ramen 8
  • The most purchased item is ramen which is a total of 8 times.

5. Which item was the most popular for each customer?

  • To rank the quantity of orders for each item from each customer, we use DENSE_RANK(). Then, we get just items with rank = 1 by using the WHERE clause.
WITH most_popular AS
(
  Select s.customer_id, m.product_name, COUNT(m.product_id) AS product_count,
  DENSE_RANK() OVER (PARTITION BY s.customer_id ORDER BY COUNT(m.product_id) DESC) AS ranked
  From sales s
  Join menu m
    ON s.product_id = m.product_id
	Group by s.customer_id, m.product_name
	)
Select customer_id, product_name,product_count
From most_popular
Where ranked = '1'

Answer

customer_id product_name order_count
A ramen 3
B sushi 2
B curry 2
B ramen 2
C ramen 3

The results are:

  • Ramen was popular with customer A.
  • Sushi, curry and ramen were popular with customer B.
  • Ramen was popular with customer C.

6. Which item was purchased first by the customer after they became a member?

  • Because we want to know what items members have bought after becoming members, use a CTE to allow us to rank members by the date of their orders.
  • In the CTE, join the menu and members tables to the sales table.
  • Filter where rank = 1 to get only the most popular product for each customer.
WITH first_item AS
(
  Select s.customer_id, m.product_name, s.order_date, e.join_date,
  DENSE_RANK() OVER (PARTITION BY s.customer_id ORDER BY s.order_date) AS ranked
  From sales s
  Join menu m
    ON m.product_id = s.product_id
  Join members e
    ON s.customer_id = e.customer_id
	Where s.order_date >= e.join_date
	)
Select customer_id, product_name
From first_item
Where ranked = '1'

Answer

customer_id product_name
A curry
B sushi

From the results, customer C is not a member.

  • Customer A ordered curry after becoming a member.
  • Customer B ordered sushi after becoming a member.

7. Which item was purchased just before the customer became a member?

  • The query for this is similar to the previous one.
  • All we need to do is change the operator >= to < so that order_date is less than join_date.
WITH before_membership AS
(
  Select s.customer_id, m.product_name, s.order_date, e.join_date,
  DENSE_RANK() OVER (PARTITION BY s.customer_id ORDER BY s.order_date) AS ranked
  From sales s
  Join menu m
    ON m.product_id = s.product_id
  Join members e
    ON s.customer_id = e.customer_id
	Where s.order_date < e.join_date
	)
Select customer_id, product_name
From before_membership
Where ranked = '1'

Answer

customer_id product_name
A sushi
A curry
C curry

We already know customer C is not a member, our results include customer A and B.

  • Customer A ordered curry before becoming a member.
  • Customer B ordered sushi before becoming a member.

8. What is the total items and amount spent for each member before they became a member?

  • The COUNT(DISTINCT) is used to count the number of purchased items.
  • SUM to get the total sum of the price of the purchaed items just before they became a member.
Select s.customer_id, COUNT( DISTINCT s.product_id) AS count_product ,SUM (m.price) AS total_sales
  From sales s
  Join menu m
    ON m.product_id = s.product_id
  Join members e
    ON s.customer_id = e.customer_id
	Where s.order_date < e.join_date
Group by  s.customer_id

Answer

customer_id count_product total_sales
A 2 25
B 2 40
  • Customer A bought 2 items and spent $25.
  • Customer B bought 3 items and spent $40.

9. If each $1 spent equates to 10 points and sushi has a 2x points multiplier - how many points would each customer have?

  • For each $1 spent, 10 points are earned.
  • However, sushi gets double the points, so for $1 = 20 points.
  • Create conditional statements by using CASE WHEN.
  • SUM function to get the total points.
WITH cust_points AS
 (
 Select s.customer_id, m.product_id,
    CASE WHEN m.product_id = 1 THEN m.price * 20
	  ELSE m.price * 10
	  END points
     From sales s
     Join menu m
        ON s.product_id = m.product_id
		)
Select customer_id, SUM(points) AS total_points
From cust_points 
Group by customer_id

Answer

customer_id total_points
A 860
B 940
C 360
  • Total points for customer A, B and C are 860, 940 and 360 respectively.

10. In the first week after a customer joins the program (including their join date) they earn 2x points on all items, not just sushi - how many points do customer A and B have at the end of January?

  • First, create a CTE to get the dates needed fo the query. Find the validity date which is 6 days after join_date and it includes the join_date with the last day as '2021-01-31'
  • Use CASE WHEN to allocate points by product_name and the dates created.
WITH Dates AS
   (Select *,
      DATEADD(DAY, 6, join_date) AS interval_dates,
      EOMONTH('2021-01-31') AS last_day
        From members)
Select d.customer_id, d.join_date, d.interval_dates, d.last_day,
  m.product_name, m.price, s.order_date,
   SUM(CASE WHEN m.product_name = 'sushi' THEN m.price * 2 * 10
            WHEN s.order_date BETWEEN d.join_date AND d.interval_dates THEN m.price * 2 * 10
			  ELSE m.price * 10
			    END) AS total_points				
From sales s
Join Dates d
  ON s.customer_id = d.customer_id
Join menu m
  ON s.product_id = m.product_id
Where s.order_date < d.interval_dates
Group by d.customer_id, d.join_date, d.interval_dates, d.last_day,
  m.product_name, m.price, s.order_date

Answer

customer_id total_points
A 1370
B 820

Customer A has 1370 points while customer B has 820 points.

BONUS QUESTIONS

Join all the things.

  • Create a new table using customer_id, order_date, product_name, price.
  • Use CASE WHEN to create a new column that states whether a customer is a member in respect to date.
Select s.customer_id, s.order_date, m.product_name, m.price,
  CASE WHEN s.order_date < e.join_date THEN 'N'
       WHEN s.order_date >= e.join_date THEN 'Y'
	     ELSE 'N'
		 END member
From sales s
Left Join menu m
  ON s.product_id = m.product_id
Left Join  members e
   ON s.customer_id = e.customer_id

Answer:

customer_id order_date product_name price member
A 2021-01-01 sushi 10 N
A 2021-01-01 curry 15 N
A 2021-01-07 curry 15 Y
A 2021-01-10 ramen 12 Y
A 2021-01-11 ramen 12 Y
A 2021-01-11 ramen 12 Y
B 2021-01-01 curry 15 N
B 2021-01-02 curry 15 N
B 2021-01-04 sushi 10 N
B 2021-01-11 sushi 10 Y
B 2021-01-16 ramen 12 Y
B 2021-02-01 ramen 12 Y
C 2021-01-01 ramen 12 N
C 2021-01-01 ramen 12 N
C 2021-01-07 ramen 12 N

Ranking of customer products

WITH product_ranking AS
(
  Select s.customer_id, s.order_date, m.product_name, m.price,
  CASE WHEN s.order_date < e.join_date THEN 'N'
       WHEN s.order_date >= e.join_date THEN 'Y'
	     ELSE 'N'
		 END member
From sales s
Left Join menu m
  ON s.product_id = m.product_id
Left Join  members e
   ON s.customer_id = e.customer_id
   )
Select *,
  CASE WHEN member = 'N' THEN NULL
    ELSE 
	  DENSE_RANK() OVER (PARTITION BY customer_id, member ORDER BY order_date)END AS ranking
From product_ranking

Answer:

customer_id order_date product_name price member ranking
A 2021-01-01 sushi 10 N NULL
A 2021-01-01 curry 15 N NULL
A 2021-01-07 curry 15 Y 1
A 2021-01-10 ramen 12 Y 2
A 2021-01-11 ramen 12 Y 3
A 2021-01-11 ramen 12 Y 3
B 2021-01-01 curry 15 N NULL
B 2021-01-02 curry 15 N NULL
B 2021-01-04 sushi 10 N NULL
B 2021-01-11 sushi 10 Y 1
B 2021-01-16 ramen 12 Y 2
B 2021-02-01 ramen 12 Y 3
C 2021-01-01 ramen 12 N NULL
C 2021-01-01 ramen 12 N NULL
C 2021-01-07 ramen 12 N NULL

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Case study solutions for #8weekstudychallenge. 🍽 danny's dinner.

https://8weeksqlchallenge.com/case-study-1/

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