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Grokking SQL for Tech InterviewsInterview Problems: Hard12. Generate the Invoice

Problem

Table: Products

+-------------+------+
| Column Name | Type |
+-------------+------+
| product_id  | int  |
| price       | int  |
+-------------+------+
product_id contains unique values.
Each row in this table shows the ID of a product and the price of one unit.

Table: Purchases

+-------------+------+
| Column Name | Type |
+-------------+------+
| invoice_id  | int  |
| product_id  | int  |
| quantity    | int  |
+-------------+------+
(invoice_id, product_id) is the primary key (combination of columns with unique values) for this table.
Each row in this table shows the quantity ordered from one product in an invoice.

Problem Definition

Write a solution to show the details of the invoice with the highest price. If two or more invoices have the same price, return the details of the one with the smallest invoice_id.

Return the result table in any order.

Example

1706040506181917 Image scaled to 80%

Output

1706040529409748 Image scaled to 35%

Try It Yourself

Database Exercise

Database Schema:

-- Database schema would be rendered here

Exercise Script:

-- Exercise script would be rendered here

Available actions: Execute

Solution

To identify the invoice with the highest total price, and in the event of a tie, select the one with the smallest invoice_id, we can follow a systematic approach using SQL’s aggregation and window functions. The solution involves calculating the total price for each invoice, determining the highest total price, handling ties by selecting the smallest invoice_id, and finally retrieving the detailed information of the selected invoice.

Approach Overview

  1. Join Purchases with Products to Calculate Subtotal per Product:

    • Combine the Purchases and Products tables to access the price of each product.
    • Calculate the subtotal for each product in an invoice by multiplying price by quantity.

  2. Aggregate Subtotals to Determine Total Price per Invoice:

    • Sum the subtotals for each invoice_id to obtain the total price of each invoice.

  3. Identify the Invoice with the Highest Total Price:

    • Determine the maximum total price across all invoices.
    • In cases where multiple invoices share this maximum total price, select the one with the smallest invoice_id.

  4. Retrieve Detailed Information of the Selected Invoice:

    • Fetch the product_id, quantity, and subtotal (price) for each product within the selected invoice.

SQL Query

WITH details_by_invoices AS
(
    SELECT     
        pu.invoice_id,
        pu.product_id,
        pu.quantity,
        pu.quantity * pr.price AS price
    FROM       
        Purchases AS pu
    INNER JOIN 
        Products AS pr
    ON         
        pu.product_id = pr.product_id 
), 
invoice_with_max_total AS
(
    SELECT   
        invoice_id
    FROM     
        details_by_invoices
    GROUP BY 
        invoice_id
    ORDER BY 
        SUM(price) DESC,
        invoice_id ASC 
    LIMIT 1
)
SELECT 
    product_id,
    quantity,
    price
FROM   
    details_by_invoices
WHERE  
    invoice_id IN
    (
        SELECT invoice_id
        FROM invoice_with_max_total
    );

Step-by-Step Approach

Step 1: Join Purchases with Products and Calculate Subtotal per Product (details_by_invoices)

Combine the Purchases and Products tables to calculate the subtotal for each product within an invoice.

SQL Snippet:

WITH details_by_invoices AS
(
    SELECT     
        pu.invoice_id,
        pu.product_id,
        pu.quantity,
        pu.quantity * pr.price AS price
    FROM       
        Purchases AS pu
    INNER JOIN 
        Products AS pr
    ON         
        pu.product_id = pr.product_id 
)
SELECT * FROM details_by_invoices;

Explanation:

  • INNER JOIN:

    • Combines each purchase (pu) with its corresponding product (pr) based on product_id.

  • Calculate Subtotal (price):

    • Multiplies quantity by price to compute the subtotal for each product in an invoice.

  • Common Table Expression (CTE) details_by_invoices:

    • Stores the resulting data for further processing.

Intermediate Output After Step 1 (details_by_invoices):

+------------+------------+----------+-------+
| invoice_id | product_id | quantity | price |
+------------+------------+----------+-------+
| 1          | 1          | 2        | 200   |
| 3          | 2          | 1        | 200   |
| 2          | 2          | 3        | 600   |
| 2          | 1          | 4        | 400   |
| 4          | 1          | 10       | 1000  |
+------------+------------+----------+-------+

Step 2: Aggregate Subtotals to Determine Total Price per Invoice

Calculate the total price for each invoice by summing the subtotals of all products within that invoice.

SQL Snippet:

WITH details_by_invoices AS
(
    SELECT     
        pu.invoice_id,
        pu.product_id,
        pu.quantity,
        pu.quantity * pr.price AS price
    FROM       
        Purchases AS pu
    INNER JOIN 
        Products AS pr
    ON         
        pu.product_id = pr.product_id 
), 
invoice_with_max_total AS
(
    SELECT   
        invoice_id
    FROM     
        details_by_invoices
    GROUP BY 
        invoice_id
)

Explanation:

  • GROUP BY invoice_id:

    • Aggregates all products within the same invoice.

  • SUM(price) AS total_price:

    • Calculates the total price for each invoice by summing the subtotals of its products.

  • Common Table Expression (CTE) invoice_with_max_total:

    • Identifies the invoice with the highest total price, handling ties by selecting the smallest invoice_id.

Intermediate Output After Step 2:

+------------+-------------+
| invoice_id | total_price |
+------------+-------------+
| 1          | 200         |
| 2          | 1000        |
| 3          | 200         |
| 4          | 1000        |
+------------+-------------+

Step 3: Identify the Invoice with the Highest Total Price (invoice_with_max_total)

Determine which invoice has the highest total_price. In the case of a tie, select the invoice with the smallest invoice_id.

SQL Snippet:

WITH details_by_invoices AS
(
    SELECT     
        pu.invoice_id,
        pu.product_id,
        pu.quantity,
        pu.quantity * pr.price AS price
    FROM       
        Purchases AS pu
    INNER JOIN 
        Products AS pr
    ON         
        pu.product_id = pr.product_id 
), 
invoice_with_max_total AS
(
    SELECT   
        invoice_id
    FROM     
        details_by_invoices
    GROUP BY 
        invoice_id
    ORDER BY 
        SUM(price) DESC,
        invoice_id ASC 
    LIMIT 1
)
SELECT * FROM invoice_with_max_total;

Explanation:

  • ORDER BY SUM(price) DESC, invoice_id ASC:

    • Sorts the invoices first by descending total_price to prioritize higher totals.
    • In case of a tie in total_price, sorts by ascending invoice_id to select the smallest ID.

  • LIMIT 1:

    • Ensures that only the top invoice is selected based on the sorting criteria.

Intermediate Output After Step 3 (invoice_with_max_total):

+------------+
| invoice_id |
+------------+
| 2          |
+------------+

Step 4: Retrieve Detailed Information of the Selected Invoice

Fetch the product_id, quantity, and subtotal (price) for each product within the selected invoice.

SQL Snippet:

WITH details_by_invoices AS
(
    SELECT     
        pu.invoice_id,
        pu.product_id,
        pu.quantity,
        pu.quantity * pr.price AS price
    FROM       
        Purchases AS pu
    INNER JOIN 
        Products AS pr
    ON         
        pu.product_id = pr.product_id 
), 
invoice_with_max_total AS
(
    SELECT   
        invoice_id
    FROM     
        details_by_invoices
    GROUP BY 
        invoice_id
    ORDER BY 
        SUM(price) DESC,
        invoice_id ASC 
    LIMIT 1
)
SELECT 
    product_id,
    quantity,
    price
FROM   
    details_by_invoices
WHERE  
    invoice_id IN
    (
        SELECT invoice_id
        FROM invoice_with_max_total
    )
ORDER BY 
    product_id;

Explanation:

  • WHERE invoice_id IN (SELECT invoice_id FROM invoice_with_max_total):

    • Filters the details_by_invoices to include only the selected invoice.

  • ORDER BY product_id:

    • Sorts the output by product_id for organized presentation.

Final Output:

+------------+----------+-------+
| product_id | quantity | price |
+------------+----------+-------+
| 1          | 4        | 400   |
| 2          | 3        | 600   |
+------------+----------+-------+

Explanation of Output:

  • Product 1:

    • Quantity: 4
    • Price per unit: $100
    • Subtotal: 4 Ă— $100 = $400

  • Product 2:

    • Quantity: 3
    • Price per unit: $200
    • Subtotal: 3 Ă— $200 = $600

  • Total for Invoice 2: $400 + $600 = $1000

Note:

  • Invoice 4 also has a total_price of $1000, but Invoice 2 is selected because it has the smaller invoice_id.
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