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Stopping recursion

Learn when a recursive CTE stops.

Stopping recursion

In a recursive CTE the condition goes in the recursive part's WHERE. It stops when the condition fails.

SELECT n + 1 FROM countdown WHERE n < 5

This query stops when n reaches 5.

n + 1 is selected from countdown. WHERE keeps only the rows where n < 5 holds.

Stopping recursion — details

Without a condition, infinite recursion occurs. Databases usually guard with a depth limit.

You put the stop condition in the WHERE of the recursive part (e.g. WHERE n < 5). In PostgreSQL, forgetting it can make the query run until memory runs out.

Practice Stopping recursion

Ensure stopping with a WHERE n < 5 condition in the recursive CTE.

Starter code

query.sql
-- stop condition
SELECT n + 1 FROM countdown

Sample solution

query.sql
SELECT n + 1 FROM countdown WHERE n < 5

Expected output

n
2
3
4
5

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Add WHERE

Question 
SELECT n + 1 FROM countdown ____ n < 5
Answer 
SELECT n + 1 FROM countdown WHERE n < 5

Explanation: WHERE n < 5 is the stop condition; once it fails the recursion produces no new rows.

2 Stopping recursion: match the terms

Term Meaning
condition stop
WHERE filter
infinite danger
limit depth

3 When does a recursive CTE stop?

  • Never
  • Always
  • When the condition is no longer met Correct answer
  • Automatically

Explanation: It stops when the condition fails.

All lessons in Advanced

1 CTE

  1. What is a CTE?
  2. Using WITH
  3. Multiple CTEs
  4. CTE vs subquery
  5. CTE with JOIN
  6. Summary - CTE

2 Recursive CTE

  1. What is a recursive CTE?
  2. Anchor and recursive parts
  3. Walk a hierarchy
  4. Tracking depth
  5. Stopping recursion
  6. Summary - Recursive CTE

3 Window functions

  1. What is a window function?
  2. ROW_NUMBER
  3. RANK and DENSE_RANK
  4. LAG and LEAD
  5. PARTITION BY
  6. Summary - Window functions

4 Project: Analytics report

  1. Analytics design
  2. CTE summary
  3. Top users with ROW_NUMBER
  4. Growth with LAG
  5. Final report

5 Pivot and unpivot

  1. What is pivot?
  2. Pivot with CASE
  3. FILTER clause
  4. Unpivot
  5. Dynamic pivot
  6. Summary - Pivot

6 Materialized view

  1. What is a materialized view?
  2. CREATE
  3. REFRESH
  4. View vs materialized view
  5. Index on materialized views
  6. Summary - Materialized view

7 Stored procedure

  1. What is a procedure?
  2. CREATE PROCEDURE
  3. Parameters
  4. CALL
  5. Control flow
  6. Summary - Stored procedure

8 Function

  1. What is a function?
  2. CREATE FUNCTION
  3. RETURN
  4. Calling a function
  5. Function vs procedure
  6. Summary - Function

9 Trigger

  1. What is a trigger?
  2. BEFORE and AFTER
  3. INSERT trigger
  4. UPDATE trigger
  5. Write an audit trigger
  6. Summary - Trigger

10 Transaction isolation

  1. What is isolation?
  2. READ UNCOMMITTED
  3. READ COMMITTED
  4. REPEATABLE READ
  5. SERIALIZABLE
  6. Summary - Isolation

11 Locking

  1. What is a lock?
  2. Shared and exclusive locks
  3. Deadlock
  4. SELECT FOR UPDATE
  5. Lock timeout
  6. Summary - Locking

12 Query optimization

  1. What is EXPLAIN?
  2. Seq scan vs index scan
  3. Cost
  4. Sargable
  5. N+1 problem
  6. Summary - Optimization

13 Partitioning

  1. What is partitioning?
  2. Range partition
  3. List partition
  4. Hash partition
  5. Partition pruning
  6. Summary - Partitioning

14 Advanced final

  1. Hierarchy report
  2. Window analysis
  3. Procedure and trigger
  4. Optimization