Understanding Recursive Queries in SQL: A Deep Dive

Introduction

Recursive queries in SQL can be challenging to understand and implement, especially when dealing with complex hierarchies. In this article, we will explore how to use recursive queries to solve a specific problem involving two tables: empleados (employees) and ventas (sales).

The goal is to calculate the sum of all sales made by employees who report directly or indirectly to main managers. We will break down the solution step-by-step, explaining key concepts and providing examples.

Background

Before diving into the solution, let’s review some fundamental concepts:

• Foreign Keys: A foreign key in a database is a field that refers to the primary key of another table. In this case, we have foreign keys gerenteId in the empleados table and empleadoId in the ventas table.
• Hierarchies: A hierarchy is a concept where one entity is subordinate to another. In our scenario, employees report directly or indirectly to main managers.

Recursive Query Basics

A recursive query uses a self-referential join to traverse a hierarchical structure. The basic syntax of a recursive query involves:

1. Defining a common table expression (CTE) that serves as the starting point for the recursion.
2. Joining the CTE with the main table using a foreign key or another suitable relationship.
3. Selecting data from the joined tables.

Step 1: Define the Recursive Query

Let’s analyze the original query provided by the user and identify areas for improvement:

• The query uses two tables: empleados and ventas.
• It applies a recursive join using CTE_org, which traverses the employee hierarchy.
• However, the query has several issues:
  • It incorrectly joins with the sales table.
  • It filters by cte.gerenteId IS NULL, which doesn’t accurately identify main managers.

Step 2: Refine the Recursive Query

We can improve upon the original query by refining it as follows:

• Simplify the recursive join: Instead of using a complex CTE with multiple joins, we can simplify the query by defining two separate CTEs:
  • cte_main_managers: This CTE identifies main managers (those with no direct supervisor).
  • cte_employees: This CTE traverses the employee hierarchy to calculate sales for each employee.
• Join with the sales table: We’ll join the refined CTEs with the sales table using a foreign key relationship.

Step 3: Calculate Total Sales

To calculate the total sales for main managers, we can use the following steps:

1. Join the refined CTEs with the sales table.
2. Group the results by main manager (identified in cte_main_managers) and sum their sales.

Example Query

Here’s a rewritten example query that demonstrates the improved approach:

WITH cte_main_managers AS (
    SELECT id, nombre, gerenteid
    FROM empleados e
    WHERE gerenteid IS NULL
), 
cte_employees AS (
    SELECT e.id, e.nombre, e.gerenteid,
           (SELECT COUNT(s.orderValue) FROM sales s WHERE e.id = s.empleadoId) as total_sales
    FROM empleados e
    LEFT JOIN cte_main_managers m ON e.gerenteid = m.id
    GROUP BY e.id, e.nombre, e.gerenteid
)
SELECT m.id, SUM(e.total_sales) as total_sales
FROM cte_main_managers m
JOIN cte_employees e ON m.id = e.gerenteid
GROUP BY m.id;

Conclusion

In this article, we have explored how to use recursive queries in SQL to solve a specific problem involving two tables: empleados and ventas. We refined the original query by simplifying the recursive join and joining with the sales table. Additionally, we demonstrated how to calculate total sales for main managers.

By understanding recursive queries and applying these concepts to your own problems, you’ll become proficient in solving complex hierarchical data challenges using SQL.

Last modified on 2024-07-13