CTE vs subquery vs view decision in SQL - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

When to use a CTE instead of a subquery?

Which situation best justifies using a Common Table Expression (CTE) instead of a subquery in SQL?

A When you want to reuse the same result multiple times within a query for better readability.

B When you need to permanently store the result for future queries.

C When you want to create an index on the result set.

D When you want to avoid using temporary tables for performance reasons.

💡 Hint

Think about readability and reusability inside a single query.

A CTE is useful when you want to write a query once and reuse its result multiple times within the same query, improving readability and maintainability.

❓ query_result

intermediate

Output difference between subquery and view

Given a table Employees with columns id, name, and department, what is the output of the following queries?

Query 1 (using subquery):

SELECT name FROM Employees WHERE department = (SELECT department FROM Employees WHERE id = 3);

Query 2 (using view):

CREATE VIEW DeptView AS SELECT department FROM Employees WHERE id = 3;
SELECT name FROM Employees WHERE department = (SELECT department FROM DeptView);

Assuming employee with id=3 belongs to 'Sales' department, what will both queries return?

A Both queries return names of all employees in the 'Sales' department.

B Query 1 returns names in 'Sales', Query 2 returns an error because views cannot be used in WHERE.

C Query 1 returns only the employee with id=3, Query 2 returns all employees in 'Sales'.

D Both queries return an empty set because subqueries and views behave differently.

💡 Hint

Think about how subqueries and views work in filtering data.

Both queries filter employees by the department of employee with id=3, which is 'Sales'. Both return all employees in 'Sales'. Views can be used in subqueries.

📝 Syntax

advanced

Identify the correct syntax for CTE usage

Which of the following SQL statements has correct syntax when defining a CTE?

SQL

WITH SalesCTE AS (SELECT * FROM Sales WHERE amount > 1000) SELECT * FROM SalesCTE;

A WITH SalesCTE (SELECT * FROM Sales WHERE amount > 1000) SELECT * FROM SalesCTE;

B WITH SalesCTE AS (SELECT * FROM Sales WHERE amount > 1000) SELECT * FROM SalesCTE;

C WITH SalesCTE AS SELECT * FROM Sales WHERE amount > 1000; SELECT * FROM SalesCTE;

D WITH SalesCTE AS (SELECT * FROM Sales WHERE amount > 1000); SELECT * FROM SalesCTE;

💡 Hint

Check the placement of AS and parentheses in CTE syntax.

Option B is the correct syntax for a CTE: WITH name AS (query) SELECT .... Other options miss AS, parentheses, or have misplaced semicolons.

❓ optimization

advanced

Performance impact of views vs CTEs

Which statement about performance is true when comparing views and CTEs in SQL?

A Views and CTEs both create temporary tables that slow down queries.

B CTEs are always faster than views because they are temporary and inline.

C Views always improve query performance because they store precomputed results.

D Neither views nor CTEs store data physically; performance depends on query complexity and indexing.

💡 Hint

Think about how views and CTEs are processed by the database engine.

Views and CTEs are query definitions, not physical tables. Performance depends on how the database optimizes the query and indexes available.

🔧 Debug

expert

Why does this query with a view fail?

Consider this SQL code:

CREATE VIEW RecentOrders AS SELECT * FROM Orders WHERE order_date > '2024-01-01';
SELECT customer_id, COUNT(*) FROM RecentOrders GROUP BY customer_id HAVING COUNT(*) > 5;

Why might this query fail in some SQL systems?

A Because the view does not include a GROUP BY clause, causing aggregation errors.

B Because views cannot be used in GROUP BY clauses.

C Because some SQL systems require explicit schema qualification when querying views.

D Because the view is not materialized and the query optimizer cannot handle aggregation on views.

💡 Hint

Consider database-specific requirements for referencing views.

Some SQL systems require the view to be referenced with schema name (e.g., dbo.RecentOrders). Without it, the query may fail due to object not found.

Practice

1. Which SQL feature is best used to temporarily break down a complex query into simpler parts for easier reading and maintenance?

easy

A. Common Table Expression (CTE)

B. View

C. Subquery

D. Index

Solution

1. Step 1: Understand the purpose of CTEs

   CTEs allow you to define temporary named result sets that can be referenced within a query, making complex queries easier to read and maintain.

2. Step 2: Compare with other options

   Views are permanent saved queries, subqueries are nested queries but can be harder to read, and indexes are for performance, not query structure.

3. Final Answer:

   Common Table Expression (CTE) -> Option A

4. Quick Check:

   Temporary query breakdown = CTE [OK]

Hint: Temporary query parts? Use CTEs [OK]

Common Mistakes:

• Confusing views as temporary structures
• Thinking subqueries improve readability as much as CTEs
• Assuming indexes help with query structure

2. Which of the following is the correct syntax to define a Common Table Expression (CTE) in SQL?

easy

A. CREATE VIEW cte_name AS SELECT * FROM table_name;

B. WITH cte_name AS (SELECT * FROM table_name) SELECT * FROM cte_name;

C. SELECT * FROM (SELECT * FROM table_name) AS cte_name;

D. DEFINE cte_name SELECT * FROM table_name;

Solution

1. Step 1: Recall CTE syntax

   CTEs start with the keyword WITH, followed by the CTE name, AS, and a query in parentheses.

2. Step 2: Check each option

   WITH cte_name AS (SELECT * FROM table_name) SELECT * FROM cte_name; matches the correct syntax. CREATE VIEW cte_name AS SELECT * FROM table_name; is for views, C is a subquery, and D is invalid syntax.

3. Final Answer:

   WITH cte_name AS (SELECT * FROM table_name) SELECT * FROM cte_name; -> Option B

4. Quick Check:

   CTE syntax starts with WITH [OK]

Hint: CTE always starts with WITH keyword [OK]

Common Mistakes:

• Using CREATE VIEW syntax for CTE
• Confusing subquery parentheses with CTE
• Using undefined keywords like DEFINE

3. Given the following SQL code, what will be the output?

WITH SalesCTE AS (
  SELECT ProductID, SUM(Quantity) AS TotalSold
  FROM Sales
  GROUP BY ProductID
)
SELECT * FROM SalesCTE WHERE TotalSold > 100;

medium

A. Empty result set because SalesCTE is not materialized

B. All products with total quantity sold less than 100

C. Syntax error due to missing GROUP BY

D. All products with total quantity sold greater than 100

Solution

1. Step 1: Analyze the CTE query

   The CTE calculates total quantity sold per product by grouping Sales data by ProductID and summing Quantity.

2. Step 2: Understand the main query filter

   The main query selects rows from SalesCTE where TotalSold is greater than 100, so it returns products with sales over 100 units.

3. Final Answer:

   All products with total quantity sold greater than 100 -> Option D

4. Quick Check:

   CTE filters totals > 100 = correct output [OK]

Hint: CTE output filters rows after aggregation [OK]

Common Mistakes:

• Thinking CTE is not executed or materialized
• Confusing GROUP BY usage
• Assuming syntax error without checking carefully

4. You wrote this SQL query but it throws an error:

SELECT * FROM (
  SELECT id, name FROM users
) AS user_sub WHERE user_sub.age > 30;

What is the most likely cause of the error?

medium

A. The subquery alias 'user_sub' is missing

B. The WHERE clause should be inside the subquery

C. The subquery does not include the 'age' column needed in WHERE

D. Subqueries cannot be used in FROM clause

Solution

1. Step 1: Check subquery columns

   The subquery selects only 'id' and 'name' columns, but the outer query filters on 'age' which is not selected.

2. Step 2: Understand SQL column visibility

   Since 'age' is not in the subquery result, the outer WHERE clause referencing 'age' causes an error.

3. Final Answer:

   The subquery does not include the 'age' column needed in WHERE -> Option C

4. Quick Check:

   Missing column in subquery causes error [OK]

Hint: Ensure subquery selects all columns used outside [OK]

Common Mistakes:

• Forgetting to alias subquery
• Thinking subqueries can't be in FROM
• Moving WHERE inside subquery unnecessarily

5. You want to reuse a complex query frequently in different reports without rewriting it each time. Which SQL feature should you use and why?

hard

A. Create a View because it saves the query in the database for easy reuse.

B. Use a CTE because it is permanent and can be called anytime.

C. Use a subquery because it is stored and optimized by the database.

D. Create an index because it stores the query results for reuse.

Solution

1. Step 1: Understand the need for reuse

   Reusing a complex query multiple times means you want a saved, reusable object in the database.

2. Step 2: Compare features for reuse

   Views are saved queries stored in the database and can be used repeatedly in different reports. CTEs are temporary and exist only during query execution. Subqueries are inline and not stored. Indexes improve performance but do not store queries.

3. Final Answer:

   Create a View because it saves the query in the database for easy reuse. -> Option A

4. Quick Check:

   Persistent reusable query = View [OK]

Hint: Persistent reusable query? Use a View [OK]

Common Mistakes:

• Confusing CTEs as permanent
• Thinking subqueries are stored
• Misunderstanding indexes as query storage