CTE as readable subquery replacement in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of CTE vs Subquery for Average Salary

Given the table employees with columns id, department, and salary, what is the output of the following query using a CTE that calculates the average salary per department and then selects departments with average salary above 5000?

SQL

WITH dept_avg AS (
  SELECT department, AVG(salary) AS avg_salary
  FROM employees
  GROUP BY department
)
SELECT department
FROM dept_avg
WHERE avg_salary > 5000
ORDER BY department;

A List of departments where the average salary is greater than 5000, sorted alphabetically

B List of all employees with salary above 5000, sorted by department

C List of departments with total salary above 5000, sorted alphabetically

D Syntax error due to missing GROUP BY clause

💡 Hint

Think about what the CTE calculates first and what the main query filters.

The CTE calculates the average salary per department. The main query selects departments where this average is greater than 5000. The result is a list of department names sorted alphabetically.

📝 Syntax

intermediate

Identify the Syntax Error in CTE Usage

Which option contains a syntax error in the use of a CTE?

SQL

WITH dept_count AS (
  SELECT department, COUNT(*) AS emp_count
  FROM employees
  GROUP BY department
)
SELECT * FROM dept_count WHERE emp_count > 10;

A WITH dept_count AS (SELECT department, COUNT(*) AS emp_count FROM employees GROUP BY department) SELECT * FROM dept_count WHERE emp_count > '10';

B WITH dept_count AS (SELECT department, COUNT(*) AS emp_count FROM employees GROUP BY department) SELECT * FROM dept_count WHERE emp_count > 10;

C WITH dept_count AS SELECT department, COUNT(*) AS emp_count FROM employees GROUP BY department SELECT * FROM dept_count WHERE emp_count > 10;

D ;01 > tnuoc_pme EREHW tnuoc_tped MORF * TCELES )tnemtraped YB PUORG seeyolpme MORF tnuoc_pme SA )*(TNUOC ,tnemtraped TCELES( SA tnuoc_tped HTIW

💡 Hint

Check the parentheses around the CTE definition.

Option C is missing parentheses around the CTE query, which is required syntax for CTEs.

❓ optimization

advanced

Optimizing Query with CTE vs Subquery

Consider a query that calculates total sales per region and then filters regions with sales above 10000. Which option shows the best use of a CTE to improve readability and avoid repeating the aggregation?

A WITH region_sales AS SELECT region, SUM(sales) AS total_sales FROM sales_data GROUP BY region SELECT region FROM region_sales WHERE total_sales > 10000;

B SELECT region FROM (SELECT region, SUM(sales) AS total_sales FROM sales_data GROUP BY region) WHERE total_sales > 10000;

C SELECT region FROM sales_data GROUP BY region HAVING SUM(sales) > 10000;

D WITH region_sales AS (SELECT region, SUM(sales) AS total_sales FROM sales_data GROUP BY region) SELECT region FROM region_sales WHERE total_sales > 10000;

💡 Hint

Look for correct CTE syntax and clear separation of steps.

Option D uses a CTE to calculate total sales per region once, then filters on that result, improving readability and avoiding repetition.

🔧 Debug

advanced

Debugging Incorrect CTE Result

A developer wrote this query to find departments with more than 5 employees, but it returns no rows. What is the likely cause?

SQL

WITH dept_emps AS (
  SELECT department, COUNT(employee_id) AS emp_count
  FROM employees
)
SELECT department FROM dept_emps WHERE emp_count > 5;

A Missing GROUP BY clause in the CTE causing emp_count to be a total count, not per department

B Incorrect alias name emp_count used in WHERE clause

C The WHERE clause should be HAVING clause

D CTE is missing a closing parenthesis

💡 Hint

Check if aggregation is grouped properly.

Without GROUP BY, COUNT(employee_id) returns total count for all rows, so emp_count is a single value and filtering by >5 returns no rows per department.

🧠 Conceptual

expert

Why Use CTE Instead of Subquery?

Which of the following is the best explanation for why using a CTE can be better than a subquery in complex SQL queries?

A CTEs always run faster than subqueries because they are cached automatically by the database engine.

B CTEs improve query readability by naming intermediate results and can be referenced multiple times, avoiding repetition.

C CTEs allow modifying data directly inside the query, unlike subqueries which are read-only.

D CTEs eliminate the need for indexes on tables used in the query.

💡 Hint

Think about how CTEs help organize complex queries.

CTEs make queries easier to read and maintain by breaking them into named parts and can be reused multiple times in the main query. They do not guarantee performance improvements or allow data modification.

Practice

1. What is the main benefit of using a WITH clause (CTE) instead of a nested subquery in SQL?

easy

A. It always runs faster than nested subqueries.

B. It makes the query easier to read and maintain by naming parts of the query.

C. It removes the need for any JOIN operations.

D. It automatically indexes the tables used in the query.

Solution

1. Step 1: Understand the purpose of CTEs

   CTEs (Common Table Expressions) allow you to name a subquery, making the SQL easier to read and organize.

2. Step 2: Compare with nested subqueries

   Nested subqueries can be hard to read because they are embedded inside other queries without a clear name.

3. Final Answer:

   It makes the query easier to read and maintain by naming parts of the query. -> Option B

4. Quick Check:

   CTEs improve readability = A [OK]

Hint: CTEs name subqueries for clarity and maintenance [OK]

Common Mistakes:

• Thinking CTEs always improve performance
• Believing CTEs remove the need for JOINs
• Assuming CTEs create indexes automatically

2. Which of the following is the correct syntax to start a CTE in SQL?

easy

A. WITH cte_name AS (SELECT ...) SELECT * FROM cte_name;

B. DEFINE cte_name AS (SELECT ...) SELECT * FROM cte_name;

C. CREATE CTE cte_name AS SELECT ...;

D. SELECT * FROM WITH cte_name AS (SELECT ...)

Solution

1. Step 1: Recall the standard CTE syntax

   The correct syntax starts with WITH cte_name AS (subquery), followed by the main query.

2. Step 2: Check each option

   WITH cte_name AS (SELECT ...) SELECT * FROM cte_name; correctly shows the CTE definition and usage. Options A, B, and C use invalid keywords or order.

3. Final Answer:

   WITH cte_name AS (SELECT ...) SELECT * FROM cte_name; -> Option A

4. Quick Check:

   CTE syntax starts with WITH = D [OK]

Hint: CTE always starts with WITH keyword followed by AS [OK]

Common Mistakes:

• Placing WITH after SELECT
• Using CREATE or DEFINE keywords for CTE
• Missing AS keyword after CTE name

3. Given the tables orders(order_id, customer_id, amount) and customers(customer_id, name), what will this query return?

WITH high_orders AS (
  SELECT customer_id, SUM(amount) AS total
  FROM orders
  GROUP BY customer_id
  HAVING SUM(amount) > 100
)
SELECT c.name, h.total
FROM high_orders h
JOIN customers c ON h.customer_id = c.customer_id;

medium

A. Names and total order amounts for customers with orders over 100 in total.

B. All customers with their total order amounts, regardless of amount.

C. Only customers with a single order over 100.

D. An error because CTEs cannot use GROUP BY.

Solution

1. Step 1: Analyze the CTE definition

   The CTE high_orders groups orders by customer and filters those with total amount > 100.

2. Step 2: Understand the main query

   The main query joins high_orders with customers to get customer names and their total order amounts.

3. Final Answer:

   Names and total order amounts for customers with orders over 100 in total. -> Option A

4. Quick Check:

   CTE filters totals > 100, join adds names = C [OK]

Hint: CTE filters first, main query joins for details [OK]

Common Mistakes:

• Confusing HAVING with WHERE
• Thinking it returns all customers
• Believing CTEs cannot use GROUP BY

4. Identify the error in this SQL query using a CTE:

WITH recent_orders AS (
  SELECT order_id, order_date
  FROM orders
  WHERE order_date > '2023-01-01'
)
SELECT *
FROM recent_orders
WHERE order_date < '2023-01-01';

medium

A. CTEs cannot be used with WHERE clauses inside them.

B. The CTE name cannot be used in the main query.

C. The query is missing a semicolon after the CTE definition.

D. The CTE filters orders after 2023-01-01 but main query filters before, resulting in no rows.

Solution

1. Step 1: Check the CTE filter condition

   The CTE selects orders with order_date > '2023-01-01'.

2. Step 2: Check the main query filter

   The main query filters order_date < '2023-01-01', which contradicts the CTE filter, so no rows match.

3. Final Answer:

   The CTE filters orders after 2023-01-01 but main query filters before, resulting in no rows. -> Option D

4. Quick Check:

   Conflicting filters cause empty result = B [OK]

Hint: Check filter conditions inside and outside CTE for conflicts [OK]

Common Mistakes:

• Thinking WHERE is not allowed in CTE
• Expecting semicolon after WITH clause
• Believing CTE names are invalid in main query

5. You want to find customers who placed orders totaling more than 500, but only include those customers who have placed at least 3 orders. Which CTE-based query correctly achieves this?

hard

A. WITH order_summary AS (
SELECT customer_id, COUNT(order_id) AS order_count
FROM orders
GROUP BY customer_id
HAVING COUNT(order_id) >= 3
)
SELECT * FROM order_summary WHERE total_amount > 500;

B. WITH order_summary AS (
SELECT customer_id, SUM(amount) AS total_amount
FROM orders
GROUP BY customer_id
HAVING SUM(amount) > 500
)
SELECT * FROM order_summary WHERE order_count >= 3;

C. WITH order_summary AS (
SELECT customer_id, COUNT(order_id) AS order_count, SUM(amount) AS total_amount
FROM orders
GROUP BY customer_id
HAVING SUM(amount) > 500 AND COUNT(order_id) >= 3
)
SELECT * FROM order_summary;

D. WITH order_summary AS (
SELECT customer_id, SUM(amount) AS total_amount
FROM orders
GROUP BY customer_id
)
SELECT * FROM order_summary WHERE total_amount > 500 AND order_count >= 3;

Solution

1. Step 1: Understand the requirements

   We need customers with total order amount > 500 and at least 3 orders.

2. Step 2: Check each option's logic

   WITH order_summary AS ( SELECT customer_id, COUNT(order_id) AS order_count, SUM(amount) AS total_amount FROM orders GROUP BY customer_id HAVING SUM(amount) > 500 AND COUNT(order_id) >= 3 ) SELECT * FROM order_summary; calculates both count and sum in the CTE and filters with HAVING both conditions, which is correct. Options B, C, and D try to filter outside the CTE on columns not selected or missing.

3. Final Answer:

   WITH order_summary AS ( SELECT customer_id, COUNT(order_id) AS order_count, SUM(amount) AS total_amount FROM orders GROUP BY customer_id HAVING SUM(amount) > 500 AND COUNT(order_id) >= 3 ) SELECT * FROM order_summary; -> Option C

4. Quick Check:

   Filter both conditions inside CTE HAVING = A [OK]

Hint: Use HAVING to filter grouped results inside CTE [OK]

Common Mistakes:

• Filtering aggregated columns outside CTE
• Missing COUNT or SUM in CTE select
• Using WHERE instead of HAVING for aggregates