Top 50+ Common Table Expression (CTE) Interview Questions & Answers

Learn CTEs in Depth?

If you want a complete, beginner-to-advanced explanation of CTEs with syntax, examples, performance tips and real-world use cases, check out this detailed guide. Common Table Expressions (CTE) in SQL : A Complete Guide

This resource is perfect for building a strong conceptual foundation before diving into interview questions.

1. What is a Common Table Expression (CTE)?

A Common Table Expression (CTE) is a temporary, named result set in SQL created using the WITH clause and used within a single SQL statement.

It allows you to define a query once and reference it as if it were a table, improving clarity and logical structure. A CTE exists only during the execution of the query and is automatically discarded afterward. It is commonly used to simplify complex queries, avoid repeated subqueries and handle hierarchical or recursive data.

WITH ActiveUsers AS (
    SELECT id, name
    FROM users
    WHERE is_active = 1
)
SELECT * FROM ActiveUsers;

2. Is a CTE stored permanently in the database?

No, a Common Table Expression (CTE) is not stored permanently in the database. A CTE exists only for the duration of the query in which it is defined. It is evaluated during query execution and automatically discarded once the query finishes. No data or definition is saved on disk and it does not persist beyond that single statement.

4. How is a CTE different from a subquery?

While both are used to create temporary result sets within a query, they differ primarily in readability, reusability and capability.

In short, CTEs improve clarity and maintainability, while subqueries are better suited for small, one-off logic.

5. Can a CTE be referenced multiple times?

Yes, a Common Table Expression (CTE) can be referenced multiple times within the same SQL query. Once defined using the WITH clause, the CTE behaves like a temporary table for that query, allowing you to reuse its result set in joins, filters or multiple SELECT statements without rewriting the logic. However, the CTE is only available to the single statement in which it is defined and cannot be used outside that query.

6. What is the scope of a CTE?

The scope of a Common Table Expression (CTE) is limited to the single SQL statement in which it is defined. A CTE is visible only to the query that immediately follows the WITH clause. It cannot be accessed by other queries, stored procedures or subsequent SQL statements. Once the statement finishes execution, the CTE goes out of scope and is automatically discarded.

In short, a CTE’s scope is one query only, not the session or the database.

7. Can we create indexes on a CTE?

No, you cannot create indexes on a Common Table Expression (CTE). A CTE is a temporary, logical result set, not a physical table stored in the database. Since it does not exist independently or persist beyond the execution of the query, indexes cannot be defined on it. If indexing is required for performance, you must use a temporary table or a permanent table instead.

8. Can we update data using a CTE?

Yes, you can update data using a Common Table Expression (CTE). A CTE itself does not store data, but when it is based on an updatable query (such as selecting from a single table), it can be used as the source for UPDATE, DELETE or INSERT operations. The modification is applied to the underlying base table, not to the CTE.

WITH InactiveUsers AS (
    SELECT id
    FROM users
    WHERE last_login < '2023-01-01'
)
UPDATE users
SET is_active = 0
WHERE id IN (SELECT id FROM InactiveUsers);

In short, a CTE can participate in data modification statements as a logical layer over the actual table being updated.

9. Are CTEs ANSI SQL compliant?

Yes, Common Table Expressions (CTEs) are ANSI SQL compliant.

They were introduced in the SQL:1999 standard and are supported by most modern relational database systems(RDBMS). While syntax details and advanced features (such as recursive CTEs) may vary slightly between databases, the basic WITH clause and CTE concept follow the ANSI SQL standard.

10. Can we use ORDER BY inside a CTE?

Yes, you can use ORDER BY inside a CTE, but it has limitations and no guaranteed effect unless combined with certain clauses.

In standard SQL, a CTE represents a logical result set and SQL does not guarantee row order unless ORDER BY is applied in the final query. Therefore, An ORDER BY inside a CTE is ignored unless it is paired with

• TOP (SQL Server)
• LIMIT / FETCH
• OFFSET
• window functions (like ROW_NUMBER())

WITH RankedUsers AS (
    SELECT *,
           ROW_NUMBER() OVER (ORDER BY created_date DESC) AS rn
    FROM Users
)
SELECT *
FROM RankedUsers
WHERE rn <= 10;

WITH UsersCTE AS (
    SELECT *
    FROM Users
    ORDER BY created_date
)
SELECT * FROM UsersCTE;

The ordering above is not guaranteed.

• Use ORDER BY in the outer query for guaranteed ordering.
• Use ORDER BY inside a CTE only when required for limiting or window logic.

12. Can a CTE reference another CTE?

Yes, a Common Table Expression (CTE) can reference another CTE.

Multiple CTEs can be defined in a single WITH clause, separated by commas and each CTE can reference any CTE defined before it in the same WITH block. This allows you to build complex queries step by step using layered, readable logic.

WITH Sales AS (
    SELECT region, SUM(amount) total
    FROM orders
    GROUP BY region
),
Ranked AS (
    SELECT *,
           RANK() OVER (ORDER BY total DESC) rnk
    FROM Sales
)
SELECT *
FROM Ranked
WHERE rnk <= 3;

13. How many CTEs can a query have?

A SQL query can have multiple CTEs and in practice there is no fixed ANSI-SQL limit on how many CTEs you can define. The practical limit depends on: Memory, Query complexity, Database configuration.

You can declare any number of CTEs in a single WITH clause, separated by commas, as long as the database’s internal limits (such as query size or complexity) are not exceeded. Each CTE can reference previously defined CTEs in the same WITH block.

14. What happens if a CTE name conflicts with a table name?

If a CTE name conflicts with a table name, the CTE takes precedence within the scope of the query.

When the SQL engine resolves object names, it first looks for a CTE defined in the WITH clause. If a CTE with that name exists, it is used instead of the table. Outside that query, the table remains unaffected. This is why it’s best practice to use clear, distinct CTE names to avoid confusion and accidental shadowing of tables.

15. What is a non-recursive CTE?

A non-recursive CTE is a Common Table Expression that does not reference itself. It is used to define a single, straightforward query result that runs once and produces a temporary result set for the main query. Non-recursive CTEs are mainly used to simplify complex queries, replace nested subqueries and improve readability and maintainability.

In short, a non-recursive CTE is a named, one-time query block used for organizing and structuring SQL logic without recursion.

16. When should you NOT use a CTE?

You should NOT use a CTE when it hurts performance or is unnecessary. Avoid CTEs in these situations:

• Performance-critical queries where the CTE is referenced many times and gets re-evaluated, leading to extra computation.
• When you need indexes or statistics on intermediate results CTEs don’t support indexing.
• For very simple queries where a direct query or small subquery is clearer.
• When the intermediate result must be reused across multiple statements, use a temporary or permanent table instead.

In short, don’t use a CTE if you need better performance control, indexing or persistence of intermediate data.

17. Is a CTE faster than a subquery?

A CTE is not inherently faster than a subquery.

From a performance perspective, most modern SQL optimizers treat CTEs and subqueries the same way. A CTE is mainly a readability and maintainability feature, not a performance optimization. In some databases, a CTE may be re-evaluated each time it is referenced, which can make it slower than an equivalent subquery or a temporary table in performance-critical scenarios.

In short, choose CTEs for clarity and evaluate performance with execution plans when speed is critical.

18. Can a CTE be nested?

Yes, a CTE can be nested, though this can happen in two different ways depending on your goal.

The most common way to "nest" is to define multiple CTEs in a single WITH clause, where one CTE references a previous one. This is technically a sequential dependency rather than a physical nest, but it allows for layered logic

WITH Level1 AS (
    SELECT * FROM Sales WHERE Region = 'North'
),
Level2 AS (
    -- This CTE "nests" or builds on Level1
    SELECT Salesperson, SUM(Amount) as Total 
    FROM Level1 
    GROUP BY Salesperson
)
SELECT * FROM Level2 WHERE Total > 1000;

Some database systems (like SQL Server or PostgreSQL) allow you to physically nest a WITH clause inside another WITH clause. This is usually avoided because it makes the code harder to read.

WITH Outer_CTE AS (
    WITH Inner_CTE AS (
        SELECT * FROM Employees
    )
    SELECT * FROM Inner_CTE WHERE Dept = 'IT'
)
SELECT * FROM Outer_CTE;

19. What is materialization in CTEs?

Materialization in CTEs means: the database executes the CTE’s query once, stores the resulting rows temporarily and reuses that stored result instead of recomputing the CTE every time it’s referenced in the same query. Materialization can help performance when the CTE is used multiple times, but some database engines do not materialize by default and may re-execute the CTE instead.

20. Which databases materialize CTEs?

Whether a database materializes a CTE (executes it once and stores the result in a temporary table) or inlines it (treats it like a subquery and merges it into the main query) depends heavily on the specific engine and version.

21. What is a recursive CTE?

A recursive CTE (Common Table Expression) is a special type of CTE that references itself within its own definition so that the query can repeatedly build and return additional rows until all results are obtained. It starts with a base (anchor) query, then uses a recursive query that refers back to the CTE itself on each iteration. This makes recursive CTEs particularly useful for querying hierarchical or self-referencing data (like organizational charts, tree structures or any parent-child relationship).

WITH RECURSIVE cte_name AS (
    -- 1. Anchor Member: The starting result set
    SELECT column_list
    FROM table_name
    WHERE condition_to_start

    UNION ALL

    -- 2. Recursive Member: References 'cte_name' to loop
    SELECT column_list
    FROM table_name
    INNER JOIN cte_name ON table_name.parent_id = cte_name.id
)
-- 3. Final Query: Uses the results
SELECT * FROM cte_name;

23. Employee hierarchy using recursive CTE

WITH RECURSIVE emp_tree AS (
    SELECT id, name, manager_id, 1 AS lvl
    FROM employees
    WHERE manager_id IS NULL

    UNION ALL

    SELECT e.id, e.name, e.manager_id, t.lvl + 1
    FROM employees e
    JOIN emp_tree t ON e.manager_id = t.id
)
SELECT * FROM emp_tree;

24. What happens if recursion never ends?

If recursion in a recursive CTE never ends (because there’s no proper termination condition or there’s a cycle in the data), the query fails with an error when the recursion limit is exceeded. Most SQL engines (like SQL Server) enforce a maximum recursion depth (e.g., 100 by default) when that limit is reached the execution stops and an error is raised instead of looping forever.

25. How do you limit recursion depth?

To limit recursion depth in a recursive CTE (so it doesn’t run infinitely or go too deep), you can do one of these depending on the SQL engine

• Add a level column that tracks the depth in the recursive query and use a WHERE condition to stop recursion after a certain level.
  WITH RECURSIVE emp_tree AS (
      SELECT id, name, manager_id, 1 AS lvl
      FROM employees
      WHERE manager_id IS NULL
  
      UNION ALL
  
      SELECT e.id, e.name, e.manager_id, t.lvl + 1
      FROM employees e
      JOIN emp_tree t ON e.manager_id = t.id
      WHERE t.lvl < 10  -- stop at level 10
  )
  SELECT * FROM emp_tree;

• Use a WHERE condition in the recursive member to restrict how deep recursion goes.
• Configure database recursion limits like SQL Server’s MAXRECURSION query hint to cap the number of recursive iterations allowed.
  WITH RECURSIVE emp_tree AS (
      …
  )
  SELECT *
  FROM emp_tree
  OPTION (MAXRECURSION 100);  -- limit recursion to 100 levels

26. UNION vs UNION ALL in recursive CTE

In a Recursive CTE, the choice between UNION and UNION ALL determines how the database handles duplicate rows and infinite loops during the recursion process.

27. Can recursive CTEs be used for graph traversal?

Yes , recursive CTEs can be used for graph traversal. A recursive CTE’s self-referencing structure lets SQL repeatedly follow relationships (edges) between rows (nodes), making it suitable for graph-like traversals such as finding connected components, friends-of-friends or route paths in a network stored in relational tables.

For example, to walk through graph edges defining connections, a recursive CTE repeatedly joins the CTE back to the edges table until all reachable nodes are found, just like traversing a graph.

This works for both tree-like hierarchies and more general graph structures, though performance and cycle-handling must be managed explicitly.

28. Are recursive CTEs expensive?

Yes, Recursive CTEs can be expensive. Because they operate in a loop, their performance cost scales based on the complexity of the data and how the query is written.

29. CTE + window functions example

WITH Ranked AS (
    SELECT name, score,
           ROW_NUMBER() OVER (ORDER BY score DESC) rnk
    FROM students
)
SELECT * FROM Ranked WHERE rnk <= 5;

30. Can a CTE be used inside a transaction?

Yes , a CTE can be used inside a transaction. A CTE executes as part of the SQL statement you run, and that statement runs inside the current transaction context. If you have an explicit BEGIN TRANSACTION … COMMIT/ROLLBACK, the CTE’s work is subject to that transaction’s commit or rollback just like any other query. If no transaction is explicitly open, the database treats the statement (and its CTE) as a single implicit transaction.

In short: CTEs do not break transaction boundaries. they run within the active transaction and follow its commit/rollback behavior.

31. Does a CTE guarantee execution order?

No. A CTE does not guarantee execution order or the order in which rows are processed or returned. SQL is a declarative language and the optimizer decides the actual evaluation order to produce the correct result. the CTE’s definition simply names a subquery, not a fixed execution sequence. If you need rows in a specific order, you must use an explicit ORDER BY in the final SELECT.

32. CTE vs Temporary Table

34. Can CTEs be debugged?

Not directly. You can’t step through or inspect a CTE like procedural code inside the database engine itself. There’s no native debugger that shows intermediate CTE results during execution.

To debug CTE logic, developers usually:

• Isolate and run the CTE’s query portion separately to view its output.
• Convert the CTE logic into temporary tables then query those tables step by step to inspect intermediate results.

35. Are CTEs optimized separately?

CTEs are not always optimized separately, many SQL engines optimize them as part of the overall query plan but others may materialize them and treat them as separate units.

36. PostgreSQL CTE optimization keywords

• MATERIALIZED : Forces PostgreSQL to evaluate the CTE once, store its result and treat it as an optimization fence. This can be useful when you want to avoid re-computing a complex CTE many times or control planning behavior.
• NOT MATERIALIZED : Tells PostgreSQL to inline the CTE into the outer query (like a subquery) so the planner can push filters/join conditions and optimize as a single unit, which often improves performance.

These keywords give you control over how the PostgreSQL optimizer handles CTEs, either as a separate materialized result or as an inline expression that can be further optimized.

37. Why are CTEs popular in analytics?

CTEs are widely used in analytics because they make complex analytical SQL cleaner, easier to understand and more maintainable. They let analysts break down large or multi-step queries into modular named parts, which improves clarity and reduces errors when writing and reviewing queries.

They also enable reusing intermediate logic within the same query and support recursive operations for hierarchical or layered data analysis both common needs in analytical workloads

38. Can CTEs be used in DELETE?

Yes , CTEs can be used with a DELETE statement. A CTE defined with WITH can be referenced by the DELETE that follows and the deletion will affect the underlying table(s) from which the CTE selects rows.

WITH cte AS (
    SELECT id
    FROM employees
    WHERE status = 'inactive'
)
DELETE
FROM employees
WHERE id IN (SELECT id FROM cte);

When you delete through a CTE (e.g., DELETE FROM cte WHERE …), SQL Server infers the base table and deletes matching rows from that table.

39. Can CTEs replace views?

In short: Yes, for individual queries, but no for database architecture.

A CTE can act like a temporary, inline view within a query because it lets you name a subquery and reuse it just like a view would. In that sense, you can often use a CTE instead of a view when you only need the logic for a single query.

But not fully, because views are permanent database objects. Views are stored in the database schema, can be reused across many queries and sessions and can be granted permissions, whereas CTEs exist only for the duration of one statement and disappear after that.

In short: You can use a CTE where you might use a view for one-off queries or to simplify complex SQL, but a CTE does not permanently replace a view because it isn’t stored or reusable outside that one query.

40. Are CTEs cached?

No , CTEs are not generally cached by default. A CTE is a temporary result set that exists only for the duration of one SQL query and most database engines will recompute it each time it’s referenced rather than store its results in a reusable cache. If you need persistent caching, you typically use a temporary table or materialized view instead.

41. Best use case for recursive CTE?

Recursive CTEs are ideal for traversing hierarchical or graph-like data structures stored relationally such as organizational charts (employee -> manager), Bill of Materials (parent -> child components), file/folder hierarchies and general graph traversal or dependency resolution, because they let you express repeated self-joins in a single declarative SQL query without procedural loops.

In short: recursive CTEs are best when you need to iteratively explore parent-child relationships or recursive data paths directly in SQL.

42. Best use case for non-recursive CTE?

The best use case for a non-recursive CTE is when you want to simplify a complex SQL query into clear, modular parts so it becomes easier to read, maintain and reuse within the same statement. Non-recursive CTEs are ideal for breaking down multi-step logic (like aggregations, joins, filters or calculations) into named building blocks that can be referenced later in the query without rewriting subqueries multiple times. This improves readability and maintainability, especially in analytics or reporting queries where clarity matters.

In short: use non-recursive CTEs to structure and simplify complex queries by defining reusable logical parts before the main SELECT.

43. Can CTEs improve code reviews?

Yes. CTEs improve SQL code readability and maintainability, which makes queries easier to understand and review during code reviews. By breaking complex logic into named, modular parts, CTEs reduce duplication, clarify intent and help reviewers quickly grasp each step of the query. This leads to clearer, simpler reviews and fewer misunderstandings compared to nested subqueries or monolithic SQL.

44. Should CTEs be used everywhere?

No. CTEs should not be used everywhere. CTEs are great for improving readability, organizing complex queries and simplifying logic, but they are not always the best choice in every situation.

45. Do CTEs affect locking?

No , CTEs themselves do not change or add locks. A CTE is just a logical subquery (similar to an inline view) and how locks are acquired depends on the statements and isolation level used in the query, not on the presence of the CTE. The database’s normal locking rules (shared locks for reads, exclusive locks for writes, etc.) still apply to the tables accessed by the CTE.

46. Are CTEs evaluated before WHERE?

No, a CTE does not guarantee that it is fully evaluated before the WHERE clause is applied. A CTE is essentially a named subquery (an inline temporary result set) that the optimizer can treat like a derived table. The database optimizer decides how to combine and reorder operations such as the CTE’s logic, joins and the WHERE filter when building the execution plan.

47. Can we join a CTE with itself?

Yes, you can join a CTE with itself. A CTE produces a named result set just like a temporary table or derived table, so you can reference it multiple times in the same query and join it to itself using aliases to compare or relate rows within that result set.

WITH SalesCTE AS (
    SELECT salesperson, sale_date, amount
    FROM Sales
)
SELECT
    a.salesperson AS PersonA,
    b.salesperson AS PersonB
FROM SalesCTE a
JOIN SalesCTE b
    ON a.sale_date = b.sale_date
WHERE a.amount > b.amount;

48. Can CTEs be used for pagination?

Yes, CTEs can be used for pagination by combining them with a window function like ROW_NUMBER() to assign row numbers to results and then filtering those rows in the outer query to return one “page” of data at a time. This technique works in SQL engines that support window functions and is commonly used when OFFSET/FETCH is not available or when you need precise control over row numbering.

WITH Paginated AS (
    SELECT
        ROW_NUMBER() OVER (ORDER BY SomeColumn) AS RowNum,
        *
    FROM YourTable
)
SELECT *
FROM Paginated
WHERE RowNum BETWEEN @StartRow AND @EndRow;

49. Are CTEs allowed inside views?

Yes, you can use a CTE in a view’s defining SELECT statement. In SQL Server and many other databases, a CTE defined with WITH can be placed at the start of the SELECT that the view uses and then the view returns results based on that CTE.

CREATE VIEW MyView AS
WITH cte AS (
    SELECT …
    FROM …
)
SELECT * FROM cte;

In a FAANG or high-level engineering environment, putting a CTE inside a View is the preferred way to handle multi-step logic. It makes the database much easier for other developers to read and maintain.

50. Biggest mistake candidates make?

Assuming CTEs always improve performance. Many candidates use CTEs thinking they inherently make queries faster, but CTEs are mainly for readability and structure and don’t guarantee better performance, in some cases they can even hurt it. Always check execution plans and test performance.

51. When should you not use a CTE?

Avoid CTEs when they don’t add clarity or performance, such as

• When the query is simple and a CTE adds unnecessary complexity - a basic JOIN or subquery may be clearer without a CTE.
• When you need to reuse the intermediate result in multiple statements - CTEs are limited to a single query and can’t be referenced elsewhere.
• When you need indexes, statistics or tuning on intermediate results - CTEs can’t have indexes, so for large datasets or performance-critical logic, a temporary table might be better.
• When performance suffers because the optimizer can’t push filters or reduce computation - overly complex CTEs can slow queries compared to other constructs.
• The query planner materializes the CTE unnecessarily, hurting performance

52. FAANG and top tech companies interview tip

When discussing CTEs in interview, make sure you always explain three things clearly

• Why you used a CTE : state the reason (e.g., to simplify multi-step logic, break a problem into readable parts or express recursion).
• What problem it solves : describe how the CTE makes the query clearer, avoids duplication or handles hierarchical/complex logic.
• Performance trade-offs : briefly mention how the CTE affects performance (e.g., readability vs optimizer behavior, materialization costs or lack of indexes).

Don’t just write SQL, articulate your reasoning, highlight the benefits of your approach and acknowledge any performance considerations.

In interviews, it’s valuable to demonstrate awareness of trade-offs, not just correctness. Candidates often assume CTEs always make queries faster, but the real answer involves understanding execution plans and optimization costs.