CTE referencing another CTE in SQL - Mini Project: Build & Apply

Using CTEs to Reference Another CTE in SQL

📖 Scenario: You are working with a sales database that tracks orders and customers. You want to analyze the total sales per customer and then find customers whose total sales exceed a certain amount.

🎯 Goal: Build a SQL query using two Common Table Expressions (CTEs). The first CTE calculates total sales per customer. The second CTE filters customers with total sales greater than 1000. Finally, select all columns from the second CTE.

📋 What You'll Learn

Create a CTE named CustomerSales that sums order_amount grouped by customer_id from the orders table.

Create a second CTE named HighValueCustomers that selects from CustomerSales only those customers with total sales greater than 1000.

Write a final SELECT statement that retrieves all columns from HighValueCustomers.

💡 Why This Matters

🌍 Real World

CTEs are used in real-world databases to organize complex queries into readable parts, especially when intermediate results are needed for further processing.

💼 Career

Knowing how to write and reference multiple CTEs is a valuable skill for data analysts and database developers to write efficient and maintainable SQL queries.

1

Create the first CTE to calculate total sales per customer

Write a SQL query that starts with WITH CustomerSales AS ( and inside it, select customer_id and the sum of order_amount as total_sales from the orders table grouped by customer_id. Close the CTE with a closing parenthesis.

SQL

-- Write the first CTE named CustomerSales to sum order_amount by customer_id
WITH CustomerSales AS (
    -- Your code here
)

Hint

Use WITH CustomerSales AS (SELECT customer_id, SUM(order_amount) AS total_sales FROM orders GROUP BY customer_id).

2

Create the second CTE to filter customers with total sales over 1000

Add a second CTE named HighValueCustomers after CustomerSales. Inside it, select all columns from CustomerSales where total_sales is greater than 1000. Close this CTE with a closing parenthesis.

SQL

WITH CustomerSales AS (
    SELECT customer_id, SUM(order_amount) AS total_sales
    FROM orders
    GROUP BY customer_id
),
HighValueCustomers AS (
    -- Your code here
)

Hint

Use HighValueCustomers AS (SELECT * FROM CustomerSales WHERE total_sales > 1000).

3

Write the final SELECT statement to get all high value customers

After the two CTEs, write a SELECT statement that selects all columns from HighValueCustomers.

SQL

WITH CustomerSales AS (
    SELECT customer_id, SUM(order_amount) AS total_sales
    FROM orders
    GROUP BY customer_id
),
HighValueCustomers AS (
    SELECT * FROM CustomerSales WHERE total_sales > 1000
)
-- Your code here

Hint

Use SELECT * FROM HighValueCustomers to get the filtered customers.

4

Complete the query with proper formatting

Ensure the full query includes both CTEs and the final SELECT statement exactly as follows: WITH CustomerSales AS (...), HighValueCustomers AS (...) SELECT * FROM HighValueCustomers.

SQL

WITH CustomerSales AS (
    SELECT customer_id, SUM(order_amount) AS total_sales
    FROM orders
    GROUP BY customer_id
),
HighValueCustomers AS (
    SELECT * FROM CustomerSales WHERE total_sales > 1000
)
SELECT * FROM HighValueCustomers
-- Your code here

Hint

Make sure the query includes both CTEs and the final SELECT statement in correct order.

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