Common Table Expressions (CTEs) in Snowflake - Mini Project: Build & Apply

Common Table Expressions (CTEs) in Snowflake

📖 Scenario: You are working with a sales database in Snowflake. You want to organize your SQL queries better by using Common Table Expressions (CTEs). CTEs help you break down complex queries into smaller, readable parts.

🎯 Goal: Build a SQL query using CTEs to calculate total sales per product category from a sales table.

📋 What You'll Learn

Create a CTE named category_sales that sums sales amounts grouped by product category.

Create a CTE named top_categories that selects categories with total sales greater than 1000.

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

Use exact CTE names and SQL syntax as specified.

💡 Why This Matters

🌍 Real World

CTEs help data analysts and engineers write clear and maintainable SQL queries for reporting and data transformation in cloud data warehouses like Snowflake.

💼 Career

Knowing how to use CTEs is essential for roles involving data querying, reporting, and building data pipelines in cloud environments.

1

Create the initial CTE for category sales

Write a CTE named category_sales that selects product_category and the sum of sales_amount as total_sales from the sales table, grouped by product_category.

Snowflake

-- Write the CTE named category_sales here
WITH category_sales AS (
    -- Your code here
)

Hint

Use WITH category_sales AS (SELECT ... FROM sales GROUP BY product_category) to create the CTE.

2

Add a second CTE to filter top categories

Add a second CTE named top_categories that selects all columns from category_sales where total_sales is greater than 1000.

Snowflake

WITH category_sales AS (
    SELECT product_category, SUM(sales_amount) AS total_sales
    FROM sales
    GROUP BY product_category
),
-- Write the CTE named top_categories here

Hint

Use top_categories AS (SELECT * FROM category_sales WHERE total_sales > 1000) to filter categories.

3

Write the final SELECT statement

Write a final SELECT statement that retrieves all columns from the top_categories CTE.

Snowflake

WITH category_sales AS (
    SELECT product_category, SUM(sales_amount) AS total_sales
    FROM sales
    GROUP BY product_category
),
top_categories AS (
    SELECT * FROM category_sales WHERE total_sales > 1000
)
-- Write the final SELECT statement here

Hint

Use SELECT * FROM top_categories to get the filtered results.

4

Complete the query with proper formatting

Ensure the entire query uses proper SQL formatting with line breaks and indentation as shown, including the final SELECT statement after the CTEs.

Snowflake

WITH category_sales AS (
    SELECT product_category, SUM(sales_amount) AS total_sales
    FROM sales
    GROUP BY product_category
),
top_categories AS (
    SELECT * FROM category_sales WHERE total_sales > 1000
)
SELECT * FROM top_categories
-- Ensure formatting is correct

Hint

Check that your query matches the formatting with line breaks and indentation.

Practice

1. What is the main purpose of using a WITH clause in Snowflake SQL?

easy

A. To set session variables

B. To permanently store data in a new table

C. To create a user-defined function

D. To define a temporary named result set for use in a query

Solution

1. Step 1: Understand the role of the WITH clause

   The WITH clause defines a Common Table Expression (CTE), which is a temporary named result set used within a query.

2. Step 2: Differentiate from other SQL features

   Unlike permanent tables or functions, CTEs exist only during query execution and help organize complex queries.

3. Final Answer:

   To define a temporary named result set for use in a query -> Option D

4. Quick Check:

   WITH clause = temporary named result set [OK]

Hint: WITH means temporary named query part [OK]

Common Mistakes:

• Thinking WITH creates permanent tables
• Confusing WITH with functions or variables
• Assuming WITH changes session settings

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

easy

A. CREATE CTE cte_name AS (SELECT * FROM table);

B. DEFINE cte_name SELECT * FROM table;

C. WITH cte_name AS (SELECT * FROM table)

D. BEGIN CTE cte_name SELECT * FROM table;

Solution

1. Step 1: Recall the standard CTE syntax

   In Snowflake, a CTE starts with the keyword WITH followed by the CTE name and AS, then the query in parentheses.

2. Step 2: Compare options to syntax

   WITH cte_name AS (SELECT * FROM table) matches this pattern exactly: WITH cte_name AS (SELECT * FROM table)

3. Final Answer:

   WITH cte_name AS (SELECT * FROM table) -> Option C

4. Quick Check:

   CTE syntax starts with WITH [OK]

Hint: CTE always starts with WITH keyword [OK]

Common Mistakes:

• Using CREATE instead of WITH
• Omitting AS keyword
• Using BEGIN or DEFINE which are invalid here

3. Given the query:

WITH cte AS (SELECT 1 AS num UNION ALL SELECT 2) SELECT SUM(num) FROM cte;

What is the output of this query?

medium

A. 3

B. 2

C. 1

D. Error

Solution

1. Step 1: Understand the CTE content

   The CTE named 'cte' selects two rows with values 1 and 2 under the column 'num'.

2. Step 2: Calculate the SUM of 'num'

   The main query sums the values 1 and 2, resulting in 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   1 + 2 = 3 [OK]

Hint: Sum values inside CTE then add [OK]

Common Mistakes:

• Thinking UNION ALL removes duplicates
• Confusing SUM with COUNT
• Expecting syntax error due to CTE

4. Identify the error in the following Snowflake query using a CTE:

WITH cte AS SELECT * FROM employees SELECT * FROM cte;

medium

A. Missing parentheses around the CTE query

B. CTE name cannot be 'cte'

C. CTE cannot be used in the same query

D. Missing AS keyword before SELECT in CTE definition

Solution

1. Step 1: Review CTE syntax requirements

   A CTE query must be enclosed in parentheses after the AS keyword.

2. Step 2: Check the given query

   The query lacks parentheses around the SELECT statement in the CTE definition.

3. Final Answer:

   Missing parentheses around the CTE query -> Option A

4. Quick Check:

   CTE query must be in parentheses [OK]

Hint: CTE query always inside parentheses [OK]

Common Mistakes:

• Omitting parentheses around CTE query
• Thinking CTE names are restricted
• Believing CTEs can't be reused in same query

5. You want to calculate the average salary per department using a CTE named dept_salaries that selects employee salaries and departments. Which query correctly uses the CTE to get the average salary per department?

hard

A. WITH dept_salaries AS SELECT department, salary FROM employees SELECT department, AVG(salary) FROM dept_salaries GROUP BY department;

B. WITH dept_salaries AS (SELECT department, salary FROM employees) SELECT department, AVG(salary) FROM dept_salaries GROUP BY department;

C. WITH dept_salaries (department, salary) AS (SELECT department, salary FROM employees) SELECT AVG(salary) FROM dept_salaries;

D. WITH dept_salaries AS (SELECT department, salary FROM employees) SELECT AVG(salary) FROM employees GROUP BY department;

Solution

1. Step 1: Verify correct CTE syntax and usage

   WITH dept_salaries AS (SELECT department, salary FROM employees) SELECT department, AVG(salary) FROM dept_salaries GROUP BY department; correctly defines the CTE with parentheses and AS, then uses it to select department and average salary grouped by department.

2. Step 2: Check aggregation and grouping

   WITH dept_salaries AS (SELECT department, salary FROM employees) SELECT department, AVG(salary) FROM dept_salaries GROUP BY department; groups by department and calculates AVG(salary) from the CTE, which is the intended calculation.

3. Final Answer:

   WITH dept_salaries AS (SELECT department, salary FROM employees) SELECT department, AVG(salary) FROM dept_salaries GROUP BY department; -> Option B

4. Quick Check:

   CTE with correct syntax and grouping = WITH dept_salaries AS (SELECT department, salary FROM employees) SELECT department, AVG(salary) FROM dept_salaries GROUP BY department; [OK]

Hint: Use WITH ... AS (...) then GROUP BY correctly [OK]

Common Mistakes:

• Omitting parentheses in CTE definition
• Not grouping by department when aggregating
• Using employees table instead of CTE in main query