What are recursive queries, and how can they be used?

Recursive queries are a very powerful feature in SQL, mainly used to handle hierarchical or self-referencing data structures, such as organizational charts, folder trees, bill of materials, or category hierarchies. They allow you to repeatedly process data that refers back to itself — like a parent-child relationship — until a condition is met.

In SQL Server (and most RDBMS like PostgreSQL or Oracle), this is typically implemented using a Common Table Expression (CTE) with the WITH clause.

I’ll usually explain it to interviewers like this — a recursive query has two parts:

1. An anchor query – which provides the base or starting point.
2. A recursive member – which repeatedly references the CTE itself to fetch related rows.
   Together, they keep looping until no new rows are returned.

Here’s a simple practical example I’ve used in an HR database:
Suppose we have an Employees table like this:

If I want to find all employees under “Asha,” including indirect reports, I can write:

WITH EmployeeHierarchy AS (
    -- Anchor query: get the root manager
    SELECT EmployeeID, EmployeeName, ManagerID
    FROM Employees
    WHERE EmployeeName = 'Asha'

    UNION ALL

    -- Recursive query: get direct reports of previously found employees
    SELECT e.EmployeeID, e.EmployeeName, e.ManagerID
    FROM Employees e
    INNER JOIN EmployeeHierarchy eh
        ON e.ManagerID = eh.EmployeeID
)
SELECT * FROM EmployeeHierarchy;

The recursive part keeps running — for each employee found, it looks for others who report to them, directly or indirectly — until there are no more matches.

In this example, the result would show Asha, Ravi, Priya, Suresh, and Meena, capturing the full hierarchy tree.

Where I’ve applied this in practice:
I used recursive queries in a reporting system for a retail chain to build product category hierarchies — each product could belong to a subcategory, which in turn belonged to a parent category. Using recursion, I generated breadcrumb paths like “Electronics > Mobile Phones > Android.”

Another case was generating approval hierarchies in a workflow system — to find all approvers above a specific employee until the top-level manager.

Challenges I’ve faced:

• Performance can degrade for deep hierarchies, as recursion can generate many iterations.
• Sometimes, there’s a risk of infinite recursion if data has circular references (like an employee set as their own ancestor). To handle that, I always include a safety check like OPTION (MAXRECURSION 100) in SQL Server.
• Recursive queries can be hard to debug or visualize when the data is large.

Limitations and alternatives:

• Recursive CTEs are not ideal for very deep or wide trees (thousands of levels). In such cases, using a hierarchyid data type (in SQL Server) or materialized path model can be more efficient.
• In some ETL systems, I’ve replaced recursive queries with iterative batch logic in stored procedures or temporary tables for better control.

In summary:
Recursive queries are a clean, SQL-native way to traverse hierarchical data without loops or procedural code. They’re excellent for organizational charts, directory trees, and dependency chains. While powerful, they must be handled carefully for performance and recursion depth — and sometimes complemented by precomputed hierarchies or specialized data types for large-scale systems.