CTE referencing another CTE in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a CTE named 'first_cte' that selects all columns from the 'employees' table.

SQL

WITH first_cte AS (SELECT * FROM [1]) SELECT * FROM first_cte;

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

Adepartments

Bemployees

Csalaries

Dprojects

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a table other than 'employees' will cause the query to return wrong data or error.

The CTE 'first_cte' must select from the 'employees' table, so the correct answer is 'employees'.

2fill in blank

medium

Complete the code to define a second CTE named 'second_cte' that selects only the 'id' and 'name' columns from 'first_cte'.

SQL

WITH first_cte AS (SELECT * FROM employees), second_cte AS (SELECT [1] FROM first_cte) SELECT * FROM second_cte;

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

Asalary, department

Bid, salary

Cname, department

Did, name

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting columns not present in 'first_cte' will cause errors.

The second CTE selects 'id' and 'name' columns from 'first_cte'.

3fill in blank

hard

Fix the error in the code by choosing the correct keyword to reference the first CTE inside the second CTE.

SQL

WITH first_cte AS (SELECT * FROM employees), second_cte AS (SELECT id, name FROM [1]) SELECT * FROM second_cte;

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

Afirst_cte

Bdepartments

Csecond_cte

Demployees

Attempts:

3 left

💡 Hint

Common Mistakes

Referencing the base table or wrong CTE name causes errors.

The second CTE must reference the first CTE by its name 'first_cte'.

4fill in blank

hard

Fill both blanks to complete the query that defines two CTEs where the second CTE filters employees with salary greater than 50000.

SQL

WITH first_cte AS (SELECT * FROM [1]), second_cte AS (SELECT id, name FROM first_cte WHERE [2] > 50000) SELECT * FROM second_cte;

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

Aemployees

Bdepartments

Csalary

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong table or column names causes errors or wrong results.

The first CTE selects from 'employees'. The second CTE filters where 'salary' is greater than 50000.

5fill in blank

hard

Fill all three blanks to complete the query that defines three CTEs: first selects employees, second filters by department 'Sales', third selects names and salaries above 60000.

SQL

WITH first_cte AS (SELECT * FROM [1]), second_cte AS (SELECT * FROM first_cte WHERE [2] = 'Sales'), third_cte AS (SELECT [3] FROM second_cte WHERE salary > 60000) SELECT * FROM third_cte;

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

Aemployees

Bdepartment

Cname, salary

Did, salary

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong column names or selecting wrong columns causes errors or wrong output.

The first CTE selects from 'employees'. The second filters where 'department' equals 'Sales'. The third selects 'name' and 'salary' columns.

Practice

1. What is the main purpose of referencing one CTE inside another CTE in SQL?

easy

A. To create permanent tables in the database

B. To speed up the database server automatically

C. To avoid using SELECT statements

D. To break down complex queries into smaller, easier steps

Solution

1. Step 1: Understand what CTEs do

   CTEs (Common Table Expressions) let you write parts of a query separately for clarity.

2. Step 2: See why referencing one CTE inside another helps

   Referencing one CTE inside another breaks complex logic into smaller, readable steps without repeating code.

3. Final Answer:

   To break down complex queries into smaller, easier steps -> Option D

4. Quick Check:

   CTEs help simplify queries = C [OK]

Hint: Think of CTEs as building blocks for queries [OK]

Common Mistakes:

• Confusing CTEs with permanent tables
• Believing CTEs speed up queries automatically
• Thinking CTEs replace SELECT statements

2. Which of the following is the correct syntax to reference one CTE inside another in SQL?

easy

A. WITH cte1 AS (SELECT * FROM table1), cte2 AS (SELECT * FROM cte1 WHERE id > 10) SELECT * FROM cte2;

B. WITH cte1 AS (SELECT * FROM table1) SELECT * FROM cte1; WITH cte2 AS (SELECT * FROM cte1 WHERE id > 10);

C. CREATE CTE cte1 AS SELECT * FROM table1; CREATE CTE cte2 AS SELECT * FROM cte1 WHERE id > 10;

D. SELECT * FROM cte1; WITH cte2 AS (SELECT * FROM cte1 WHERE id > 10);

Solution

1. Step 1: Review correct CTE syntax

   Multiple CTEs are defined together after WITH, separated by commas, and can reference each other.

2. Step 2: Check each option

   WITH cte1 AS (SELECT * FROM table1), cte2 AS (SELECT * FROM cte1 WHERE id > 10) SELECT * FROM cte2; correctly defines cte1, then cte2 referencing cte1, then selects from cte2. Others misuse WITH or CREATE keywords.

3. Final Answer:

   WITH cte1 AS (SELECT * FROM table1), cte2 AS (SELECT * FROM cte1 WHERE id > 10) SELECT * FROM cte2; -> Option A

4. Quick Check:

   Multiple CTEs separated by commas = A [OK]

Hint: Use one WITH clause for all CTEs, separated by commas [OK]

Common Mistakes:

• Using multiple WITH clauses separately
• Trying to CREATE CTEs like tables
• Placing SELECT before WITH

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

WITH first_cte AS (
  SELECT 1 AS num UNION ALL SELECT 2 UNION ALL SELECT 3
), second_cte AS (
  SELECT num * 10 AS val FROM first_cte WHERE num > 1
)
SELECT * FROM second_cte ORDER BY val;

medium

A. [20, 30]

B. Syntax error

C. [1, 2, 3]

D. [10, 20, 30]

Solution

1. Step 1: Analyze first_cte output

   first_cte returns rows with num = 1, 2, 3.

2. Step 2: Analyze second_cte filtering and calculation

   second_cte selects num * 10 as val where num > 1, so num=2 and 3, val=20 and 30.

3. Final Answer:

   [20, 30] -> Option A

4. Quick Check:

   Filter num > 1, multiply by 10 = [20, 30] [OK]

Hint: Trace each CTE step by step to find final output [OK]

Common Mistakes:

• Including num=1 in second_cte output
• Confusing num and val columns
• Assuming syntax error due to multiple CTEs

4. Identify the error in this SQL query using CTEs:

WITH cte1 AS (
  SELECT id, name FROM users
),
cte2 AS (
  SELECT id, name FROM cte3 WHERE id > 5
)
SELECT * FROM cte2;

medium

A. SELECT statement missing after CTEs

B. Missing comma between CTE definitions

C. cte2 references cte3 which is not defined

D. CTEs cannot reference other CTEs

Solution

1. Step 1: Check CTE definitions

   cte1 is defined, but cte2 references cte3 which is not defined anywhere.

2. Step 2: Confirm syntax correctness

   Comma between cte1 and cte2 is present, and SELECT after CTEs is correct, so no syntax error there.

3. Final Answer:

   cte2 references cte3 which is not defined -> Option C

4. Quick Check:

   Undefined CTE reference = A [OK]

Hint: Check all referenced CTE names are defined above [OK]

Common Mistakes:

• Assuming missing comma error
• Thinking CTEs can't reference others
• Overlooking undefined CTE names

5. You have two CTEs: sales_cte with columns (product_id, total_sales) and products_cte with columns (product_id, product_name). How do you write a query using CTE referencing another CTE to get product names with sales over 1000?

hard

A. WITH sales_cte AS (SELECT product_id, SUM(amount) AS total_sales FROM sales GROUP BY product_id) SELECT product_name FROM products WHERE product_id IN (SELECT product_id FROM sales_cte WHERE total_sales > 1000);

B. WITH sales_cte AS (SELECT product_id, SUM(amount) AS total_sales FROM sales GROUP BY product_id), products_cte AS (SELECT product_id, product_name FROM products), final_cte AS (SELECT p.product_name FROM products_cte p JOIN sales_cte s ON p.product_id = s.product_id WHERE s.total_sales > 1000) SELECT * FROM final_cte;

C. WITH products_cte AS (SELECT product_id, product_name FROM products), sales_cte AS (SELECT product_id, SUM(amount) AS total_sales FROM sales GROUP BY product_id) SELECT product_name FROM products_cte WHERE total_sales > 1000;

D. WITH final_cte AS (SELECT product_name FROM products JOIN sales ON products.product_id = sales.product_id WHERE sales.amount > 1000) SELECT * FROM final_cte;

Solution

1. Step 1: Define sales_cte and products_cte correctly

   sales_cte sums sales per product_id; products_cte selects product info.

2. Step 2: Create final_cte referencing both CTEs

   Join products_cte and sales_cte on product_id, filter total_sales > 1000, select product_name.

3. Step 3: Select from final_cte

   Final SELECT returns product names with sales over 1000.

4. Final Answer:

   WITH sales_cte AS (SELECT product_id, SUM(amount) AS total_sales FROM sales GROUP BY product_id), products_cte AS (SELECT product_id, product_name FROM products), final_cte AS (SELECT p.product_name FROM products_cte p JOIN sales_cte s ON p.product_id = s.product_id WHERE s.total_sales > 1000) SELECT * FROM final_cte; -> Option B

5. Quick Check:

   Layered CTEs with join and filter = B [OK]

Hint: Build CTEs stepwise, join in last CTE for filtering [OK]

Common Mistakes:

• Referencing columns not in CTEs
• Filtering before joining CTEs
• Not joining CTEs properly