Understanding SQL Queries: Combining Multiple Operations into a Single Query

As a beginner in SQLite, you have taken the first step by familiarizing yourself with basic SQL statements. However, as you delve deeper into database management, you may encounter more complex scenarios that require combining multiple operations into a single query. In this article, we will explore one such scenario where you need to select two max/min values from different columns in a single SQL query.

The Problem

Let’s assume you have a table called PRODUCTS with three columns: Product, Kcal, and Price. You want to retrieve the supermarket name along with the maximum and minimum values of Kcal and Price, respectively, for each supermarket. However, you are unsure whether it is possible to perform this operation in a single query.

Breaking Down the Problem

To understand how to approach this problem, let’s break it down into smaller sub-queries:

Sub-query 1: Finding Max Kcal

We want to find the maximum value of Kcal for each supermarket. We can achieve this using the MAX() function in SQL.

SELECT Supermarket, MAX(Kcal) AS max_kcal
FROM PRODUCTS
GROUP BY Supermarket;

Sub-query 2: Finding Min Price

We want to find the minimum value of Price for each supermarket. We can achieve this using the MIN() function in SQL.

SELECT Supermarket, MIN(Price) AS min_price
FROM PRODUCTS
GROUP BY Supermarket;

Combining Sub-queries into a Single Query

To combine these two sub-queries into a single query, we need to join the results of both queries with other tables. However, this approach may not be efficient as it requires multiple database operations.

A more efficient approach is to create two separate tables: one for max Kcal values and another for min Price values. Then, we can join these two tables with the original table using a common column like Supermarket.

Creating Separate Tables for Max Kcal and Min Price

Let’s assume we have two separate tables:

Table A (Max Kcal)

Table B (Min Price)

Joining Tables A and B with the Original Table

We can now join these two tables with the original table using a common column like Supermarket.

SELECT A.supermarket, 
       A.max_kcal, 
       C.NAME, 
       B.min_price, 
       D.NAME 
FROM   (SELECT supermarket, 
               MAX(kcal) AS max_kcal 
        FROM   product 
        GROUP  BY supermarket) AS A 
       JOIN (SELECT supermarket, 
                    MIN(price) AS min_price 
             FROM   product 
             GROUP  BY supermarket) AS B 
         ON A.supermarket = B.supermarket 
       JOIN product C 
         ON C.kcal = A.max_kcal 
            AND C.supermarket = A.supermarket 
       JOIN product D 
         ON D.price = B.min_price 
            AND D.supermarket = B.supermarket
ORDER BY A.supermarket;

Explanation

In this query, we first create two separate tables: A for max Kcal values and B for min Price values. We then join these two tables with the original table using a common column like Supermarket.

We use the MAX() function in SQL to find the maximum value of Kcal for each supermarket, and we use the MIN() function to find the minimum value of Price for each supermarket.

Finally, we join the results of both queries with other tables using the JOIN clause. We use the NAME column from the original table to get the product name.

Conclusion

In this article, we explored how to combine multiple SQL operations into a single query. By creating separate tables for max Kcal values and min Price values, we can join these two tables with the original table using a common column like Supermarket.

This approach may not be efficient as it requires multiple database operations. However, it provides a clear understanding of how to tackle complex queries.

As you continue to explore SQL, remember that practice makes perfect. Try experimenting with different queries and approaches to improve your skills.

Last modified on 2024-01-12