Recursive Join in SQL (original) (raw)

Last Updated : 19 Dec, 2024

In SQL, a **recursive join is a powerful technique used to handle **hierarchical data relationships, such as managing employee-manager relationships, family trees, or any data with a self-referential structure. This type of **join enables us to combine data from the same table repeatedly, accumulating records until no further changes are made to the result set.

In this article, we will explore recursive **joins in SQL, understand the concept of recursive common table expressions (**CTEs), and work through detailed examples to illustrate how to use recursive joins effectively.

What is a Recursive Join in SQL?

Recursive joins are implemented using recursive **common table expressions (CTEs). CTEs are temporary result sets that can be referred to within the execution scope of a **SELECT, **INSERT, **UPDATE, or DELETE statement. In a recursive CTE, a query is repeatedly executed to gather related data, making it possible to handle hierarchical relationships like parent-child data.

**Syntax:

WITH RECURSIVE cte_name AS (
-- Anchor Query: Select the root or starting point
SELECT columns
FROM table
WHERE condition

UNION ALL  
  
-- Recursive Query: Join the CTE with the table to fetch related data  
SELECT t.columns  
FROM table t  
INNER JOIN cte_name cte ON t.column = cte.column  

)

Example of Recursive Join in SQL

Let’s walk through an example where we create an **employee-manager hierarchy using a recursive join in SQL. Assume we have a table of employees where each employee has a manager_id pointing to their manager’s employee_id. The goal is to retrieve a list of employees along with their managers, all the way up the chain.

Now, we will use a recursive join to get a list of all employees and their managers, starting with Ankit (employee with employee_id = 1).

**Query:

WITH RECURSIVE employee_hierarchy AS (
-- Anchor query: Start with Ankit (employee_id = 1)
SELECT employee_id, employee_name, manager_id, age
FROM employees
WHERE employee_id = 1

UNION ALL  
  
-- Recursive query: Join the employees table with itself to get the employees reporting to each manager  
SELECT e.employee_id, e.employee_name, e.manager_id, e.age  
FROM employees e  
INNER JOIN employee_hierarchy eh ON e.manager_id = eh.employee_id  

)
SELECT * FROM employee_hierarchy;

**Output:

**Explanation:

• The anchor part selects Ankit as the starting point.
• The recursive part joins the employees table with the employee_hierarchy CTE to find all employees who report to the previous level of employees.
• The process repeats until all employees who report to Ankit (and indirectly to others) are listed.

Applications of Recursive Joins

• **Hierarchical Data Representation: Recursive joins are commonly used to represent and query hierarchical structures, such as employee-manager relationships, organizational charts, and bill of materials.
• **Parent-Child Relationships: Recursive queries help retrieve data that represents parent-child relationships, such as categories and subcategories in a product catalog.
• **Graph Traversal: Recursive joins are also used for traversing graphs or networks, such as social networks or transportation networks.

Conclusion

Recursive joins in SQL, implemented through recursive CTEs, are a vital tool for querying hierarchical data efficiently. Whether it’s navigating organizational structures, analyzing product categories, or working with parent-child relationships, recursive joins simplify the process of building and querying hierarchies. By using the WITH RECURSIVE clause and combining it with an INNER JOIN, SQL provides a powerful way to traverse and retrieve nested data.