8 Week SQL Challenge, Case Study #1, Danny's Diner

Answers.sql

-- Case Study #1 - Danny's Diner
-- https://8weeksqlchallenge.com/case-study-1/
-- Solved on SQLite 3.39 by Farhad Uneci, August 2022

-- 1. What is the total amount each customer spent at the restaurant?
SELECT
    customer_id AS 'Customer',
    SUM(price) AS 'Paid'
FROM
    sales
    JOIN menu ON sales.product_id = menu.product_id
GROUP BY
    customer_id
ORDER BY
    'Paid' DESC,
    'Customer';

-- 2. How many days has each customer visited the restaurant?
SELECT
    customer_id,
    COUNT(DISTINCT(order_date)) AS 'Visit Count'
FROM
    sales
GROUP BY
    customer_id;

-- 3. What was the first item from the menu purchased by each customer?
WITH Ranked AS (
    SELECT
        sales.customer_id AS 'Customer',
        menu.product_name AS 'Product',
        sales.order_date AS 'Date',
        DENSE_RANK() OVER (
            PARTITION BY sales.customer_id
            ORDER BY
                sales.order_date
        ) AS 'Rank'
    FROM
        sales
        JOIN menu ON sales.product_id = menu.product_id
)
SELECT
    DISTINCT Customer,
    Product,
    Date
FROM
    Ranked
WHERE
    Rank = 1;

-- 4. What is the most purchased item on the menu and how many times was it purchased by all customers?
SELECT
    menu.product_name AS 'Product',
    SUM(1) || ' ' || 'times' AS 'Sold'
FROM
    sales
    JOIN menu ON sales.product_id = menu.product_id
GROUP BY
    sales.product_id
ORDER BY
    'Sold' DESC
LIMIT
    5;

Database.sql

CREATE TABLE sales (
  "customer_id" VARCHAR(1),
  "order_date" DATE,
  "product_id" INTEGER
);

INSERT INTO sales
  ("customer_id", "order_date", "product_id")
VALUES
  ('A', '2021-01-01', '1'),
  ('A', '2021-01-01', '2'),
  ('A', '2021-01-07', '2'),
  ('A', '2021-01-10', '3'),
  ('A', '2021-01-11', '3'),
  ('A', '2021-01-11', '3'),
  ('B', '2021-01-01', '2'),
  ('B', '2021-01-02', '2'),
  ('B', '2021-01-04', '1'),
  ('B', '2021-01-11', '1'),
  ('B', '2021-01-16', '3'),
  ('B', '2021-02-01', '3'),
  ('C', '2021-01-01', '3'),
  ('C', '2021-01-01', '3'),
  ('C', '2021-01-07', '3');
 

CREATE TABLE menu (
  "product_id" INTEGER,
  "product_name" VARCHAR(5),
  "price" INTEGER
);

INSERT INTO menu
  ("product_id", "product_name", "price")
VALUES
  ('1', 'sushi', '10'),
  ('2', 'curry', '15'),
  ('3', 'ramen', '12');
  

CREATE TABLE members (
  "customer_id" VARCHAR(1),
  "join_date" DATE
);

INSERT INTO members
  ("customer_id", "join_date")
VALUES
  ('A', '2021-01-07'),
  ('B', '2021-01-09');