Recursion in SQL - Use Cases for the Recursive SQL Common Table Expression (CTE)

by Michael Bellamy

Recursion in SQL is an advanced feature of Structured Query Language (SQL), that empowers users to traverse hierarchical data structures and establish complex relationships within databases. It's a valuable tool for querying tree-like or graph-like data representations. By allowing queries to refer back to their own results, recursive SQL can uncover intricate patterns and hierarchies that are crucial for various applications, from organizational charts to social networks.

Recursion in SQL is achieved through the use of a recursive SQL common table expression (CTE). This is a query that continuously references a previous result until it returns an empty result. It’s achieved using a CTE, which in SQL is known as a “WITH” statement. This allows you to name the result and reference it within other queries later.  

Benefits of Recursion SQL:

1. Hierarchical Data Exploration: Recursion in SQL is exceptionally effective in querying and understanding hierarchical relationships within data, enabling a deep dive into structures like organizational charts, family trees, and network hierarchies.

2. Efficient Data Analysis: Recursive CTE queries streamline the process of analyzing complex relationships, making it easier to identify patterns, categorize data, and draw valuable insights from intricate datasets.

A basic example of a Recursive SQL CTE Query:

Consider a scenario where you want to generate a list of numbers from 1-10.

Or maybe you need to generate a list with the full date for each day of the year

An advanced example of Recursive SQL CTE Query:

Consider a scenario where you have a table representing an organizational structure, where each record contains information about an employee and their direct manager. The table has columns: `employee_id`, `employee_name`, and `manager_id`.

In this recursive SQL CTE query, we start with the employee named 'John Doe' and look through the data to find all the employees under John Doe's management. The query recursively joins the table with itself, following the defined hierarchy until no more relationships are found, revealing the complete employee hierarchy under John Doe.

Recursive SQLCTE queries have the ability to navigate basic tasks like generating a list of numbers, a sequence of dates, or even hierarchical data structures. It is a valuable tool in any SQL user’s toolkit, unlocking powerful insights and relationships within complex datasets.