Recursive CTE concept in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a recursive CTE with the anchor member.

SQL

WITH RECURSIVE cte AS (SELECT [1] AS n)

Drag options to blanks, or click blank then click option'

A1

Bn

C*

DCOUNT(*)

Attempts:

3 left

💡 Hint

Common Mistakes

Using a column name instead of a constant for the anchor member.

Using an aggregate function in the anchor member.

The anchor member of a recursive CTE must provide a starting value. Here, we start with 1 as the initial number.

2fill in blank

medium

Complete the code to add the recursive member that increments the number by 1.

SQL

WITH RECURSIVE cte AS (SELECT 1 AS n UNION ALL SELECT n [1] 1 FROM cte WHERE n < 5)

Drag options to blanks, or click blank then click option'

A/

B+

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition.

Forgetting to reference the CTE in the recursive member.

The recursive member adds 1 to the current number to generate the next number in the sequence.

3fill in blank

hard

Fix the error in the recursive CTE by completing the termination condition.

SQL

WITH RECURSIVE cte AS (SELECT 1 AS n UNION ALL SELECT n + 1 FROM cte WHERE n [1] 10) SELECT * FROM cte;

Drag options to blanks, or click blank then click option'

A<=

B>

C=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' which stops recursion too early.

Using '=' which only allows one iteration.

The termination condition must allow recursion until n is less than or equal to 10 to include 10 in the results.

4fill in blank

hard

Fill both blanks to create a recursive CTE that generates even numbers up to 10.

SQL

WITH RECURSIVE evens AS (SELECT 2 AS num UNION ALL SELECT num [1] 2 FROM evens WHERE num [2] 10) SELECT * FROM evens;

Drag options to blanks, or click blank then click option'

A+

B-

C<=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition.

Using a wrong comparison operator that excludes 10.

The recursive member adds 2 to generate even numbers, and the condition ensures numbers up to 10 are included.

5fill in blank

hard

Fill all three blanks to create a recursive CTE that calculates factorial of numbers from 1 to 5.

SQL

WITH RECURSIVE factorial AS (SELECT 1 AS n, 1 AS fact UNION ALL SELECT n [1] 1, fact [2] n FROM factorial WHERE n [3] 5) SELECT * FROM factorial;

Drag options to blanks, or click blank then click option'

A+

B*

C<=

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition for n.

Using addition instead of multiplication for factorial.

Using wrong comparison operator that stops recursion too early.

The recursive CTE increments n by 1, multiplies fact by n, and continues until n is 5 to compute factorials.

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