Common Table Expressions (CTEs) in Snowflake - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Common Table Expression (CTE) in Snowflake?

A CTE is a temporary named result set in SQL that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. It helps organize complex queries by breaking them into simpler parts.

beginner

How do you define a CTE in Snowflake SQL?

You start with the WITH keyword, followed by the CTE name, then AS and a query in parentheses. Example: WITH cte_name AS (SELECT * FROM table_name) SELECT * FROM cte_name;

intermediate

Can you use multiple CTEs in a single Snowflake query?

Yes, you can define multiple CTEs separated by commas after the WITH keyword. Each CTE can build on the previous ones, making complex queries easier to read and maintain.

beginner

What is one key benefit of using CTEs in Snowflake queries?

CTEs improve query readability and organization by breaking down complex queries into smaller, named parts. They also help avoid repeating the same subquery multiple times.

beginner

Are CTEs in Snowflake stored permanently in the database?

No, CTEs are temporary and exist only during the execution of the query. They do not create permanent tables or views.

What keyword starts a Common Table Expression in Snowflake SQL?

A JOIN

B SELECT

C FROM

D WITH

CTEs always start with the WITH keyword followed by the CTE name and query.

Can a CTE reference another CTE defined earlier in the same WITH clause?

A Yes, CTEs can reference previous CTEs

B No, CTEs cannot reference other CTEs

C Only if they are in different queries

D Only if they are permanent tables

CTEs can build on each other by referencing earlier CTEs in the same WITH clause.

What happens to a CTE after the query finishes executing?

A It is deleted and not stored

B It becomes a permanent table

C It is saved as a view

D It is cached for future queries

CTEs are temporary and exist only during query execution; they are not stored permanently.

Which of the following is a benefit of using CTEs?

A Increases query complexity

B Creates permanent tables

C Improves query readability

D Requires less SQL knowledge

CTEs help organize and simplify complex queries, making them easier to read.

How do you separate multiple CTEs in a single WITH clause?

A With semicolons

B With commas

C With periods

D With colons

Multiple CTEs are separated by commas within the WITH clause.

Explain what a Common Table Expression (CTE) is and how it helps in writing SQL queries in Snowflake.

Think about how you can break a big task into smaller steps.

Describe how you can use multiple CTEs in one Snowflake query and why that might be useful.

Imagine building a recipe step by step.

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