Recursive CTE concept in SQL - Mini Project: Build & Apply

Build an Employee Hierarchy Using Recursive CTE

📖 Scenario: You work in a company database where employees have managers. You want to find the full chain of command for each employee, starting from the top manager down to the employee.

🎯 Goal: Create a recursive Common Table Expression (CTE) to list each employee along with their manager hierarchy.

📋 What You'll Learn

Create a table called employees with columns id, name, and manager_id.

Insert the given employee data with exact values.

Create a recursive CTE named employee_hierarchy to find all levels of management.

Select the employee name and their manager chain from the CTE.

💡 Why This Matters

🌍 Real World

Companies often store employee-manager relationships in databases. Recursive CTEs help find full reporting chains easily.

💼 Career

Understanding recursive CTEs is important for database analysts and developers working with hierarchical data.

1

Create the employees table and insert data

Write SQL to create a table called employees with columns id (integer), name (text), and manager_id (integer). Then insert these exact rows: (1, 'Alice', NULL), (2, 'Bob', 1), (3, 'Charlie', 2), (4, 'Diana', 2), (5, 'Eve', 1).

SQL

-- Create the employees table and insert data
-- Your code here

Hint

Use CREATE TABLE to define the structure. Use INSERT INTO with multiple rows to add data.

2

Define the base query for the recursive CTE

Write the first part of the recursive CTE named employee_hierarchy that selects employees who have no manager (where manager_id is NULL). Select id, name, and set manager_chain to the employee's name.

SQL

WITH RECURSIVE employee_hierarchy AS (
  -- Base query: select employees with no manager
  SELECT id, name, name AS manager_chain
  FROM employees
  WHERE manager_id IS NULL
  -- Your code here
)

Hint

The base query finds top-level managers who have no manager themselves.

3

Add the recursive part to build the manager chain

Add the recursive SELECT to employee_hierarchy that joins employees to employee_hierarchy on employees.manager_id = employee_hierarchy.id. Select employees.id, employees.name, and concatenate employee_hierarchy.manager_chain with ' > ' and employees.name as manager_chain.

SQL

  UNION ALL
  -- Recursive query: join employees to employee_hierarchy
  SELECT employees.id, employees.name, employee_hierarchy.manager_chain || ' > ' || employees.name
  FROM employees
  JOIN employee_hierarchy ON employees.manager_id = employee_hierarchy.id
  -- Your code here
)

Hint

The recursive part adds employees under their managers, building the chain.

4

Select the full employee hierarchy from the CTE

Write a SELECT statement after the recursive CTE to get name and manager_chain from employee_hierarchy. Order the results by id ascending.

SQL

SELECT name, manager_chain
FROM employee_hierarchy
ORDER BY id ASC;
-- Your code here

Hint

This final query shows each employee with their full manager chain.

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