Why CTEs matter in PostgreSQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a Common Table Expression (CTE) in PostgreSQL.

PostgreSQL

WITH [1] AS (SELECT * FROM employees)

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

Acte

BSELECT

CTABLE

DJOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using SQL keywords like SELECT or JOIN as the CTE name.

Forgetting to name the CTE.

The keyword WITH starts a CTE, and the name after it (like cte) defines the CTE's name.

2fill in blank

medium

Complete the code to select from a CTE named 'cte' in PostgreSQL.

PostgreSQL

WITH cte AS (SELECT id, name FROM employees) SELECT [1] FROM cte;

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

A*

Bemployees

Ccte

Did, name

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting the CTE name instead of columns.

Using '*' when specific columns are expected.

After defining the CTE, you select columns from it. Here, id, name are the columns to select.

3fill in blank

hard

Fix the error in the CTE usage by completing the missing keyword.

PostgreSQL

WITH cte (id, name) [1] (SELECT id, name FROM employees) SELECT * FROM cte;

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

AAS

BFROM

CINTO

DJOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using FROM or INTO instead of AS.

Omitting the keyword entirely.

The keyword AS is required to define the CTE's query after its name and optional column list.

4fill in blank

hard

Fill both blanks to create a recursive CTE that counts down from 3 to 1.

PostgreSQL

WITH RECURSIVE countdown([1]) AS (SELECT 3 UNION ALL SELECT [2] - 1 FROM countdown WHERE [2] > 1) SELECT * FROM countdown;

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

An

Ccount

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the column in the CTE and recursive SELECT.

Using SQL keywords as column names.

The recursive CTE uses the column name n and refers to it in the recursive SELECT.

5fill in blank

hard

Fill all three blanks to create a CTE that filters employees with salary over 50000 and selects their names and salaries.

PostgreSQL

WITH high_salary AS (SELECT [1], [2] FROM employees WHERE [3] > 50000) SELECT * FROM high_salary;

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

Aname

Bsalary

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting wrong columns like id instead of name.

Using incorrect column in WHERE clause.

The CTE selects name and salary columns, filtering where salary is greater than 50000.

Practice

1. What is the main purpose of using a Common Table Expression (CTE) in PostgreSQL?

easy

A. To create indexes automatically

B. To break complex queries into simpler, readable parts

C. To permanently store data in the database

D. To replace all JOIN operations

Solution

1. Step 1: Understand what a CTE does

   A CTE allows you to write a temporary named result set that you can use within a query, making complex queries easier to manage.

2. Step 2: Compare options to CTE purpose

   Options A, B, and D describe other database features, not the main purpose of CTEs.

3. Final Answer:

   To break complex queries into simpler, readable parts -> Option B

4. Quick Check:

   CTEs simplify complex queries = C [OK]

Hint: CTEs simplify complex queries by naming parts [OK]

Common Mistakes:

• Thinking CTEs store data permanently
• Confusing CTEs with indexes
• Assuming CTEs replace JOINs entirely

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

easy

A. WITH cte_name AS (SELECT * FROM table_name)

B. CREATE CTE cte_name SELECT * FROM table_name

C. DEFINE cte_name AS SELECT * FROM table_name

D. START CTE cte_name (SELECT * FROM table_name)

Solution

1. Step 1: Recall CTE syntax

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

2. Step 2: Check each option

   Only WITH cte_name AS (SELECT * FROM table_name) matches the correct syntax. Options A, C, and D use invalid keywords or structure.

3. Final Answer:

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

4. Quick Check:

   CTE starts with WITH ... AS (...) = B [OK]

Hint: CTEs always start with WITH keyword [OK]

Common Mistakes:

• Using CREATE instead of WITH
• Omitting AS keyword
• Not enclosing query in parentheses

3. Given the query:

WITH recent_orders AS (SELECT * FROM orders WHERE order_date > CURRENT_DATE - INTERVAL '7 days') SELECT customer_id FROM recent_orders;

What does this query return?

medium

A. An error because CTE is not allowed here

B. All orders from the last 7 days

C. All customers in the database

D. Customer IDs who placed orders in the last 7 days

Solution

1. Step 1: Analyze the CTE definition

   The CTE named recent_orders selects all orders where the order_date is within the last 7 days.

2. Step 2: Analyze the main query

   The main query selects customer_id from recent_orders, so it returns customer IDs who placed orders in the last 7 days.

3. Final Answer:

   Customer IDs who placed orders in the last 7 days -> Option D

4. Quick Check:

   CTE filters recent orders, main query selects customer_id = A [OK]

Hint: CTE defines data, main query selects from it [OK]

Common Mistakes:

• Thinking it returns all orders instead of customer IDs
• Assuming it returns all customers
• Believing CTE causes syntax error here

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

WITH cte AS SELECT * FROM users SELECT * FROM cte;

medium

A. Missing parentheses around the CTE query

B. CTE name cannot be 'cte'

C. SELECT * FROM cte is invalid syntax

D. WITH keyword should be lowercase

Solution

1. Step 1: Check CTE syntax

   The CTE query must be enclosed in parentheses after AS. The query misses parentheses around SELECT * FROM users.

2. Step 2: Validate other parts

   CTE name 'cte' is valid, SELECT * FROM cte is correct, and WITH keyword case does not matter.

3. Final Answer:

   Missing parentheses around the CTE query -> Option A

4. Quick Check:

   CTE query must be in parentheses = D [OK]

Hint: Always put CTE query inside parentheses [OK]

Common Mistakes:

• Omitting parentheses around CTE query
• Thinking CTE names are restricted
• Confusing case sensitivity of WITH

5. You want to optimize a complex report query by breaking it into steps using CTEs. Which of these is a key advantage of using CTEs for performance in PostgreSQL?

hard

A. CTEs automatically create indexes on intermediate results

B. CTEs always cache intermediate results to speed up queries

C. CTEs allow controlling query execution order and readability

D. CTEs reduce disk space usage by compressing data

Solution

1. Step 1: Understand CTE behavior in PostgreSQL

   CTEs help organize queries into logical steps, improving readability and sometimes controlling execution order.

2. Step 2: Evaluate performance claims

   CTEs do not always cache results (PostgreSQL inlines them unless MATERIALIZED is used), do not create indexes automatically, nor compress data.

3. Final Answer:

   CTEs allow controlling query execution order and readability -> Option C

4. Quick Check:

   CTEs improve readability and control execution = A [OK]

Hint: CTEs improve readability and control execution order [OK]

Common Mistakes:

• Assuming CTEs always cache results
• Thinking CTEs create indexes automatically
• Believing CTEs compress data to save space