Get aggregated average values joining three tables and display them next to each value in first table

Question

I have three tables which you can also find in the SQL fiddle:

CREATE TABLE Sales (
    Product_ID VARCHAR(255),
    Sales_Value VARCHAR(255),
    Sales_Quantity VARCHAR(255)
);
INSERT INTO Sales
(Product_ID, Sales_Value, Sales_Quantity)
VALUES 
("P001", "500", "200"),
("P002", "600", "100"),
("P003", "300", "250"),
("P004", "900", "400"),
("P005", "800", "600"),
("P006", "200", "150"),
("P007", "700", "550");


CREATE TABLE Products (
    Product_ID VARCHAR(255),
    Product_Name VARCHAR(255),
    Category_ID VARCHAR(255)
);
INSERT INTO Products
(Product_ID, Product_Name, Category_ID)
VALUES 
("P001", "Shirt", "C001"),
("P002", "Dress", "C001"),
("P003", "Hoodie", "C002"),
("P004", "Ball", "C002"),
("P005", "Ski", "C002"),
("P006", "Boot", "C003"),
("P007", "Flip-Flop", "C003");


CREATE TABLE Categories (
    Category_ID VARCHAR(255),
    Category_Name VARCHAR(255)
);
INSERT INTO Categories
(Category_ID, Category_Name)
VALUES 
("C001", "Fashion"),
("C002", "Sport"),
("C003", "Shoes");

The first table contains the Sales for each product.
The second table contains details about each product.
The third table contains categories.

---

Now, I want to display all products and the average_sales_price_per_category next to each product.
The result should look like this:

Product_ID      Category      average_sales_price_per_category
P001             Fashion               3.66
P002             Fashion               3.66
P003             Sport                 1.60
P004             Sport                 1.60
P005             Sport                 1.60
P006             Shoes                 1.28
P007             Shoes                 1.28

I tried to go with the solution from this question but I get an Error:

SELECT s.Product_ID, c.Category_Name,
       (SELECT SUM(SS.Sales_Value) / SUM(SS.Sales_Quantity)
        FROM Sales SS 
        WHERE SS.Category_ID = S.Category_ID
       ) AS average_sales_price
FROM Sales s 
JOIN Products p ON p.Product_ID = s.Product_ID
JOIN Categories c ON c.Category_ID = p.Category_ID;

Error

Unknown column 'SS.Category_ID' in 'where clause'

What do I need to change in my code to get the expected result?

Answer 1

You s table is not visible in inner subquery for avoid the where condition in inner subquery you could use a join on the grouped aggreated subquery

SELECT
s.Product_ID,
Price_Category.average_sales_price_per_category
FROM Sales s
JOIN Products p ON p.Product_ID = s.Product_ID
JOIN
  (SELECT 
  c.Category_ID,
  c.Category_Name,
  SUM(s.Sales_Value) / SUM(s.Sales_Quantity) AS average_sales_price_per_category
  FROM Sales s 
  JOIN Products p ON p.Product_ID = s.Product_ID
  JOIN Categories c ON c.Category_ID = p.Category_ID
  GROUP BY 1) Price_Category ON Price_Category.Category_ID = p.Category_ID;

SQL Fiddle

Answer 2

Firstly, you don't have any Category_Id column within the Sales table.

Replace your JOIN clauses with LEFT JOINs against probability of non-matched cases, and add GROUP BY c.Category_Name, c.Category_ID at the end :

SELECT c.Category_Name,c.Category_ID,
       SUM(S.Sales_Value) / SUM(S.Sales_Quantity) AS average_sales_price       
  FROM Sales s 
  LEFT JOIN Products p ON p.Product_ID = s.Product_ID
  LEFT JOIN Categories c ON c.Category_ID = p.Category_ID
 GROUP BY c.Category_Name, c.Category_ID;

where a correlated subquery is not needed.

Demo

Answer 3

The problem with your query is that categories are not available at sales level.

If you are running MySQL 8.0, you can combine aggregation and window functions as follows:

SELECT 
    p.Product_ID, 
    c.Category_Name,
    SUM(SUM(s.Sales_Value)) OVER(PARTITION BY c.Category_ID)
        / SUM(SUM(s.Sales_Quantity)) OVER(PARTITION BY c.Category_ID)
       AS average_sales_price
FROM Sales s 
JOIN Products p ON p.Product_ID = s.Product_ID
JOIN Categories c ON c.Category_ID = p.Category_ID
GROUP BY p.Product_ID, c.Category_ID, c.Category_Name

Demo on DB Fiddle:

| Product_ID | Category_Name | average_sales_price |
| ---------- | ------------- | ------------------- |
| P001       | Fashion       | 3.6666666666666665  |
| P002       | Fashion       | 3.6666666666666665  |
| P003       | Sport         | 1.6                 |
| P004       | Sport         | 1.6                 |
| P005       | Sport         | 1.6                 |
| P006       | Shoes         | 1.2857142857142858  |
| P007       | Shoes         | 1.2857142857142858  |