Recursive CTE for graph traversal in PostgreSQL - Mini Project: Build & Apply

Recursive CTE for Graph Traversal

📖 Scenario: You are working with a company's organizational chart stored in a database. Each employee has a manager, and you want to find all employees under a specific manager, including indirect reports.

🎯 Goal: Build a recursive SQL query using a Common Table Expression (CTE) to find all employees reporting directly or indirectly to a given manager.

📋 What You'll Learn

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

Insert the given employee data with their managers

Write a recursive CTE named subordinates to find all employees under a specific manager

Select all employee names and their levels under the manager

💡 Why This Matters

🌍 Real World

Companies often store organizational charts in databases. Recursive queries help find all employees under a manager, useful for reporting and permissions.

💼 Career

Understanding recursive CTEs is important for database developers and analysts working with hierarchical data like org charts, file systems, or category trees.

1

Create the employees table and insert data

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', 1), (4, 'David', 2), (5, 'Eve', 2).

PostgreSQL

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

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows.

2

Set the starting manager ID

Create a variable or parameter called start_manager_id and set it to 1. This will be the manager whose subordinates we want to find.

PostgreSQL

CREATE TABLE employees (
    id INTEGER PRIMARY KEY,
    name TEXT NOT NULL,
    manager_id INTEGER
);

INSERT INTO employees (id, name, manager_id) VALUES
(1, 'Alice', NULL),
(2, 'Bob', 1),
(3, 'Charlie', 1),
(4, 'David', 2),
(5, 'Eve', 2);

-- Set the starting manager ID
-- Your code here

Hint

Use \set start_manager_id 1 in psql to define a variable.

3

Write the recursive CTE subordinates

Write a recursive CTE named subordinates that starts with employees whose manager_id equals :start_manager_id. Then recursively find employees whose manager_id is in the previous level. Include columns id, name, and level (starting at 1 for direct reports).

PostgreSQL

CREATE TABLE employees (
    id INTEGER PRIMARY KEY,
    name TEXT NOT NULL,
    manager_id INTEGER
);

INSERT INTO employees (id, name, manager_id) VALUES
(1, 'Alice', NULL),
(2, 'Bob', 1),
(3, 'Charlie', 1),
(4, 'David', 2),
(5, 'Eve', 2);

\set start_manager_id 1

WITH RECURSIVE subordinates AS (
    -- Your code here
)

Hint

Start with direct reports, then join recursively on manager_id.

4

Select all subordinates with their levels

Write a SELECT statement to get name and level from the subordinates CTE. Order the results by level ascending and then by name ascending.

PostgreSQL

CREATE TABLE employees (
    id INTEGER PRIMARY KEY,
    name TEXT NOT NULL,
    manager_id INTEGER
);

INSERT INTO employees (id, name, manager_id) VALUES
(1, 'Alice', NULL),
(2, 'Bob', 1),
(3, 'Charlie', 1),
(4, 'David', 2),
(5, 'Eve', 2);

\set start_manager_id 1

WITH RECURSIVE subordinates AS (
    SELECT id, name, 1 AS level
    FROM employees
    WHERE manager_id = :start_manager_id
    UNION ALL
    SELECT e.id, e.name, s.level + 1
    FROM employees e
    INNER JOIN subordinates s ON e.manager_id = s.id
)

-- Your code here

Hint

Select from the CTE and order by level and name.

Practice

1. What is the main purpose of using a WITH RECURSIVE clause in PostgreSQL when working with graph data?

easy

A. To optimize query performance by indexing graph nodes

B. To create temporary tables for storing graph data

C. To repeatedly execute a query that references itself to explore connected nodes

D. To join two unrelated tables without conditions

Solution

1. Step 1: Understand the purpose of recursive CTEs

   Recursive CTEs allow a query to call itself repeatedly, which is useful for exploring connected data like graph nodes.

2. Step 2: Identify the correct use case

   Using WITH RECURSIVE lets you traverse graph structures step-by-step by referencing the CTE inside itself.

3. Final Answer:

   To repeatedly execute a query that references itself to explore connected nodes -> Option C

4. Quick Check:

   Recursive CTE = self-referencing query [OK]

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

Common Mistakes:

• Confusing recursive CTE with temporary tables
• Thinking it automatically optimizes performance
• Using it for unrelated table joins

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

easy

A. WITH path RECURSIVE AS (SELECT ... UNION ALL SELECT ... FROM path ...)

B. CREATE RECURSIVE TABLE path AS SELECT ...

C. SELECT RECURSIVE * FROM path

D. WITH RECURSIVE path AS (SELECT ... UNION ALL SELECT ... FROM path ...)

Solution

1. Step 1: Recall the syntax for recursive CTEs

   The correct syntax starts with WITH RECURSIVE cte_name AS ( ... ), where the CTE references itself inside.

2. Step 2: Match the syntax options

   WITH RECURSIVE path AS (SELECT ... UNION ALL SELECT ... FROM path ...) correctly uses WITH RECURSIVE path AS (SELECT ... UNION ALL SELECT ... FROM path ...). Other options misuse keywords or order.

3. Final Answer:

   WITH RECURSIVE path AS (SELECT ... UNION ALL SELECT ... FROM path ...) -> Option D

4. Quick Check:

   WITH RECURSIVE + CTE name + self-reference [OK]

Hint: Start with WITH RECURSIVE cte_name AS (...) [OK]

Common Mistakes:

• Swapping RECURSIVE and CTE name order
• Using CREATE instead of WITH
• Omitting UNION ALL between parts

3. Given the table edges(source INT, target INT) with data:
(1, 2), (2, 3), (3, 4), (2, 5)
What will the following query return?

WITH RECURSIVE path AS (SELECT source, target FROM edges WHERE source = 1 UNION ALL SELECT e.source, e.target FROM edges e JOIN path p ON e.source = p.target) SELECT * FROM path ORDER BY source, target;

medium

A. Rows: (1,2), (2,3), (2,5), (3,4)

B. Rows: (1,2), (2,3), (3,4)

C. Rows: (1,2), (2,5)

D. Empty result

Solution

1. Step 1: Identify the anchor and recursive parts

   The anchor selects edges where source=1: (1,2). The recursive part joins edges where source matches previous target.

2. Step 2: Trace recursive expansion

   From (1,2), next edges with source=2 are (2,3) and (2,5). From (2,3), next edge with source=3 is (3,4). No further edges from 5 or 4.

3. Final Answer:

   Rows: (1,2), (2,3), (2,5), (3,4) -> Option A

4. Quick Check:

   Recursive join expands all reachable edges [OK]

Hint: Follow edges from start node stepwise [OK]

Common Mistakes:

• Ignoring multiple edges from same source
• Stopping recursion too early
• Misordering results

4. Consider this recursive CTE:

WITH RECURSIVE path(node) AS (SELECT 1 UNION ALL SELECT node + 1 FROM path WHERE node < 3) SELECT * FROM path;

What is wrong if this query runs forever without stopping?

medium

A. Not selecting from the CTE in the final SELECT

B. Missing a proper termination condition in the recursive part

C. Using UNION instead of UNION ALL

D. Incorrect anchor query syntax

Solution

1. Step 1: Analyze the recursive condition

   The recursive part adds 1 to node while node < 3, but due to UNION ALL accumulating duplicates of rows satisfying the condition, recursion continues indefinitely.

2. Step 2: Identify why it loops infinitely

   If the termination condition is incorrect or not reached, recursion never stops. Here, the condition is present but ineffective against duplicates, causing infinite loop.

3. Final Answer:

   Missing a proper termination condition in the recursive part -> Option B

4. Quick Check:

   Infinite recursion = no stop condition [OK]

Hint: Check recursive WHERE clause stops recursion [OK]

Common Mistakes:

• Confusing UNION and UNION ALL effects
• Ignoring anchor query correctness
• Forgetting to select from CTE

5. You have a table employees(id INT, manager_id INT) representing a company hierarchy. How would you write a recursive CTE to find all employees under manager with id = 10, including indirect reports?

hard

A. WITH RECURSIVE subordinates AS (
SELECT id FROM employees WHERE manager_id = 10
UNION ALL
SELECT e.id FROM employees e JOIN subordinates s ON e.manager_id = s.id
) SELECT * FROM subordinates;

B. WITH RECURSIVE subordinates AS (
SELECT id FROM employees WHERE id = 10
UNION ALL
SELECT e.id FROM employees e JOIN subordinates s ON e.id = s.manager_id
) SELECT * FROM subordinates;

C. WITH RECURSIVE subordinates AS (
SELECT id FROM employees WHERE manager_id = 10
UNION ALL
SELECT e.manager_id FROM employees e JOIN subordinates s ON e.id = s.id
) SELECT * FROM subordinates;

D. WITH RECURSIVE subordinates AS (
SELECT id FROM employees WHERE manager_id = 10
UNION ALL
SELECT e.id FROM employees e JOIN subordinates s ON e.id = s.manager_id
) SELECT * FROM subordinates;

Solution

1. Step 1: Identify the anchor query

   The anchor selects employees directly reporting to manager 10: WHERE manager_id = 10.

2. Step 2: Understand the recursive join

   The recursive part joins employees whose manager_id matches any previously found employee id, thus finding indirect reports.

3. Final Answer:

   WITH RECURSIVE subordinates AS ( SELECT id FROM employees WHERE manager_id = 10 UNION ALL SELECT e.id FROM employees e JOIN subordinates s ON e.manager_id = s.id ) SELECT * FROM subordinates; -> Option A

4. Quick Check:

   Join on e.manager_id = s.id to find subordinates [OK]

Hint: Join on manager_id = previous employee id to find reports [OK]

Common Mistakes:

• Swapping join conditions (id vs manager_id)
• Starting anchor with wrong WHERE clause
• Selecting wrong columns in recursive part