Recursive CTE concept in SQL

Introduction

Recursive CTE helps you repeat a query step by step until you get all related results. It is like following a chain of links one by one.

Finding all managers above an employee in a company hierarchy.

Listing all parts needed to build a product from smaller parts.

Exploring family trees to find ancestors or descendants.

Calculating factorial or Fibonacci numbers step by step in SQL.

Syntax

SQL

WITH RECURSIVE cte_name (column_list) AS (
  -- Anchor member: starting query
  SELECT initial_columns
  FROM table_name
  WHERE condition
  
  UNION ALL
  
  -- Recursive member: refers to cte_name
  SELECT next_columns
  FROM table_name
  JOIN cte_name ON join_condition
  WHERE recursive_condition
)
SELECT * FROM cte_name;

The first SELECT is the starting point (anchor).

The second SELECT repeats using results from the first, building step by step.

Examples

This example counts from 1 to 5 by adding 1 each time.

SQL

WITH RECURSIVE numbers(n) AS (
  SELECT 1
  UNION ALL
  SELECT n + 1 FROM numbers WHERE n < 5
)
SELECT * FROM numbers;

This shows what happens if the anchor returns no rows: the whole CTE returns no rows.

SQL

WITH RECURSIVE empty_cte(n) AS (
  SELECT 1 WHERE false
  UNION ALL
  SELECT n + 1 FROM empty_cte WHERE n < 3
)
SELECT * FROM empty_cte;

Here the recursion stops immediately because the condition is false at start.

SQL

WITH RECURSIVE single_node(n) AS (
  SELECT 10
  UNION ALL
  SELECT n + 1 FROM single_node WHERE n < 10
)
SELECT * FROM single_node;

Sample Program

This query finds the chain of employees managed by employee 1, showing each level down.

SQL

WITH RECURSIVE employee_hierarchy(employee_id, manager_id, level) AS (
  -- Anchor: start with employee 1
  SELECT employee_id, manager_id, 1 AS level
  FROM employees
  WHERE employee_id = 1
  
  UNION ALL
  
  -- Recursive: find employee managed by current employee
  SELECT e.employee_id, e.manager_id, eh.level + 1
  FROM employees e
  JOIN employee_hierarchy eh ON e.manager_id = eh.employee_id
)
SELECT * FROM employee_hierarchy ORDER BY level;

Important Notes

Recursive CTEs run until no new rows are added, so be careful to avoid infinite loops.

Time complexity depends on how many levels or rows the recursion goes through.

Common mistake: forgetting the anchor query or the stopping condition causes infinite recursion.

Use recursive CTEs when you need to explore hierarchical or linked data step by step.

Summary

Recursive CTEs let you repeat queries stepwise to explore linked data.

They have an anchor (start) and recursive part (repeat).

Useful for hierarchies, chains, and stepwise calculations.

Practice

1. What is the main purpose of a Recursive CTE in SQL?

easy

A. To create temporary tables for one-time use

B. To join two unrelated tables

C. To optimize query performance by indexing

D. To repeatedly execute a query to process hierarchical or linked data

Solution

1. Step 1: Understand Recursive CTE definition

   A Recursive CTE runs a query repeatedly, starting from an anchor and then recursively processing related rows.

2. Step 2: Identify its main use

   This is mainly used to explore hierarchical or linked data stepwise, like organizational charts or folder trees.

3. Final Answer:

   To repeatedly execute a query to process hierarchical or linked data -> Option D

4. Quick Check:

   Recursive CTE = repeated query for linked data [OK]

Hint: Recursive CTEs repeat queries to explore linked data [OK]

Common Mistakes:

• Confusing recursive CTE with simple temporary tables
• Thinking recursive CTEs optimize indexes
• Assuming recursive CTEs join unrelated tables

2. Which of the following is the correct basic syntax to start a recursive CTE in SQL?

easy

A. WITH cte_name AS (SELECT ... UNION ALL SELECT ... FROM cte_name ...)

B. CREATE RECURSIVE cte_name AS SELECT ...

C. SELECT * FROM cte_name RECURSIVE

D. WITH RECURSIVE cte_name SELECT ... UNION SELECT ...

Solution

1. Step 1: Recall recursive CTE syntax

   The standard syntax starts with WITH cte_name AS (anchor_query UNION ALL recursive_query).

2. Step 2: Check options for correct form

   WITH cte_name AS (SELECT ... UNION ALL SELECT ... FROM cte_name ...) correctly shows the anchor and recursive parts combined with UNION ALL and references the CTE name in the recursive part.

3. Final Answer:

   WITH cte_name AS (SELECT ... UNION ALL SELECT ... FROM cte_name ...) -> Option A

4. Quick Check:

   Recursive CTE syntax = WITH + UNION ALL + self-reference [OK]

Hint: Recursive CTE uses WITH + UNION ALL + self-reference [OK]

Common Mistakes:

• Using CREATE instead of WITH
• Omitting UNION ALL
• Not referencing the CTE name recursively

3. Given the table Employees(id, manager_id) and this recursive CTE:

WITH RECURSIVE EmpCTE AS (
  SELECT id, manager_id FROM Employees WHERE id = 1
  UNION ALL
  SELECT e.id, e.manager_id FROM Employees e JOIN EmpCTE c ON e.manager_id = c.id
)
SELECT id FROM EmpCTE ORDER BY id;

What does this query return?

medium

A. All employees who have no manager

B. All employees who report directly or indirectly to employee with id 1

C. Only the employee with id 1

D. All employees in the table

Solution

1. Step 1: Analyze the anchor query

   The anchor selects employee with id = 1.

2. Step 2: Analyze the recursive part

   The recursive part joins Employees where manager_id matches the id from previous results, thus finding direct reports, then their reports, and so on.

3. Final Answer:

   All employees who report directly or indirectly to employee with id 1 -> Option B

4. Quick Check:

   Recursive CTE finds all subordinates under id 1 [OK]

Hint: Recursive join on manager_id finds all reports [OK]

Common Mistakes:

• Thinking it returns only the anchor row
• Assuming it returns employees without managers
• Believing it returns all employees regardless of manager

4. Identify the error in this recursive CTE:

WITH RECURSIVE Numbers AS (
  SELECT 1 AS num
  UNION ALL
  SELECT num + 1 FROM Numbers WHERE num < 5
)
SELECT * FROM Numbers;

medium

A. Missing anchor query

B. No termination condition in recursive part

C. Recursive CTE syntax is correct and will run successfully

D. Using UNION instead of UNION ALL causes infinite loop

Solution

1. Step 1: Check anchor query

   The anchor selects 1 AS num, which is correct.

2. Step 2: Check recursive part and termination

   The recursive part adds 1 to num and stops when num reaches 5, so it has a termination condition.

3. Final Answer:

   Recursive CTE syntax is correct and will run successfully -> Option C

4. Quick Check:

   Anchor + recursive with termination = valid recursive CTE [OK]

Hint: Check for termination condition and correct UNION ALL usage [OK]

Common Mistakes:

• Confusing UNION and UNION ALL effects
• Missing termination condition causing infinite recursion
• Assuming anchor query is missing

5. You want to calculate the factorial of 5 using a recursive CTE. Which query correctly computes this?

hard

A. WITH RECURSIVE Fact(n, fact) AS (
SELECT 1, 1
UNION ALL
SELECT n + 1, fact * (n + 1) FROM Fact WHERE n < 5
)
SELECT fact FROM Fact WHERE n = 5;

B. WITH Fact AS (
SELECT 1 AS n, 1 AS fact
UNION
SELECT n + 1, fact * n FROM Fact WHERE n <= 5
)
SELECT fact FROM Fact WHERE n = 5;

C. WITH RECURSIVE Fact(n, fact) AS (
SELECT 1, 1
UNION ALL
SELECT n + 1, fact * n FROM Fact WHERE n < 5
)
SELECT fact FROM Fact WHERE n = 5;

D. WITH RECURSIVE Fact(n, fact) AS (
SELECT 0, 1
UNION ALL
SELECT n + 1, fact * n FROM Fact WHERE n < 5
)
SELECT fact FROM Fact WHERE n = 5;

Solution

1. Step 1: Understand factorial calculation

   Factorial of n is product of all integers from 1 to n. We start with n=1, fact=1.

2. Step 2: Check recursive step correctness

   WITH RECURSIVE Fact(n, fact) AS ( SELECT 1, 1 UNION ALL SELECT n + 1, fact * (n + 1) FROM Fact WHERE n < 5 ) SELECT fact FROM Fact WHERE n = 5; correctly increments n by 1 and multiplies fact by (n+1), matching factorial definition up to 5.

3. Step 3: Verify termination and final selection

   It stops when n reaches 5 and selects fact at n=5, giving 120.

4. Final Answer:

   Option A correctly computes factorial of 5 using recursive CTE -> Option A

5. Quick Check:

   Factorial recursion: multiply by next number, stop at 5 [OK]

Hint: Multiply by n+1 each step, stop at target number [OK]

Common Mistakes:

• Starting from 0 causing multiplication by zero
• Using UNION instead of UNION ALL
• Incorrect multiplication order in recursive step