Why CTEs are needed in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a CTE named 'recent_orders' that selects orders from 2023.

SQL

WITH [1] AS (SELECT * FROM orders WHERE order_date >= '2023-01-01') SELECT * FROM recent_orders;

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

Arecent_orders

Borders_2022

Cold_orders

Dall_orders

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different CTE name than the one referenced in the main query.

Forgetting to name the CTE.

The CTE name must match the name used later in the query. Here, 'recent_orders' is the correct name.

2fill in blank

medium

Complete the code to select customer_id and total from the CTE named 'customer_totals'.

SQL

WITH customer_totals AS (SELECT customer_id, SUM(amount) AS total FROM sales GROUP BY customer_id) SELECT [1] FROM customer_totals;

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

Acustomer_id, total

B*

Camount

Dcustomer_name

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting columns not defined in the CTE.

Using SELECT * when specific columns are asked.

You need to select the columns defined in the CTE: customer_id and total.

3fill in blank

hard

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

SQL

WITH recent_orders [1] (SELECT * FROM orders WHERE order_date >= '2023-01-01') SELECT * FROM recent_orders;

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

AWHERE

BSELECT

CAS

DFROM

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the AS keyword causes syntax errors.

Using SELECT or FROM instead of AS after the CTE name.

The keyword 'AS' is required after the CTE name to define the CTE query.

4fill in blank

hard

Fill both blanks to create a CTE that calculates average sales and then selects customers with sales above 1000.

SQL

WITH avg_sales AS (SELECT customer_id, AVG(amount) [1] sales_avg FROM sales GROUP BY customer_id) SELECT customer_id, sales_avg FROM avg_sales WHERE sales_avg [2] 1000;

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

AAS

B>

C<

DSUM

Attempts:

3 left

💡 Hint

Common Mistakes

Using SUM instead of AVG for average calculation.

Using < instead of > in the WHERE clause.

The alias keyword 'AS' names the average column, and '>' filters customers with sales above 1000.

5fill in blank

hard

Fill all three blanks to create a CTE that finds top products by total sales and selects those with sales over 5000.

SQL

WITH top_products AS (SELECT product_id, [1](quantity * price) [2] total_sales FROM sales GROUP BY product_id) SELECT product_id, total_sales FROM top_products WHERE total_sales [3] 5000;

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

ASUM

BAS

C>

DAVG

Attempts:

3 left

💡 Hint

Common Mistakes

Using AVG instead of SUM for total sales.

Omitting AS keyword.

Using < instead of > in the WHERE clause.

SUM calculates total sales, AS names the column, and > filters products with sales over 5000.

Practice

1. Why do we use Common Table Expressions (CTEs) in SQL queries?

easy

A. To speed up the database server hardware

B. To break complex queries into simpler, readable parts

C. To store data permanently like tables

D. To create indexes automatically

Solution

1. Step 1: Understand the purpose of CTEs

   CTEs help organize complex SQL queries by dividing them into smaller, manageable parts.

2. Step 2: Compare other options

   Options B, C, and D describe unrelated database functions like hardware speed, permanent storage, and indexing, which are not the purpose of CTEs.

3. Final Answer:

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

4. Quick Check:

   CTEs simplify queries = A [OK]

Hint: CTEs make big queries easier to read and manage [OK]

Common Mistakes:

• Thinking CTEs improve hardware speed
• Confusing CTEs with permanent tables
• Assuming CTEs create indexes automatically

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

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 standard CTE syntax

   The correct way to define a CTE is using the WITH keyword followed by the CTE name and AS, then the query in parentheses.

2. Step 2: Check other options for syntax errors

   Options A, C, and D use incorrect keywords like CREATE, DEFINE, or START which are not valid for CTEs.

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 SalesCTE AS (SELECT salesperson, SUM(amount) AS total_sales FROM sales GROUP BY salesperson) SELECT * FROM SalesCTE WHERE total_sales > 1000;
What does this query return?

medium

A. Error because CTE cannot be filtered

B. All sales records with amount greater than 1000

C. Sum of all sales amounts in the sales table

D. Salespersons with total sales greater than 1000

Solution

1. Step 1: Analyze the CTE definition

   The CTE named SalesCTE groups sales by salesperson and calculates total sales per person.

2. Step 2: Understand the main query filter

   The main query selects only those salespersons whose total sales exceed 1000.

3. Final Answer:

   Salespersons with total sales greater than 1000 -> Option D

4. Quick Check:

   CTE groups and filters salespersons by total sales > 1000 = A [OK]

Hint: CTEs can be filtered in main query like tables [OK]

Common Mistakes:

• Thinking it returns individual sales over 1000
• Assuming it sums all sales without grouping
• Believing CTEs cannot be filtered

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

WITH cte AS SELECT id, name FROM users SELECT * FROM cte;

medium

A. Missing parentheses around the CTE query

B. CTE name cannot be 'cte'

C. SELECT * FROM cte should come before WITH clause

D. No error, query is correct

Solution

1. Step 1: Check CTE syntax

   The CTE query must be enclosed in parentheses after AS keyword.

2. Step 2: Verify query order and naming

   The CTE name 'cte' is valid, and the main SELECT comes after the CTE definition, so no error there.

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 after AS [OK]

Common Mistakes:

• Omitting parentheses around CTE query
• Placing main SELECT before WITH clause
• Thinking CTE names are restricted

5. You want to find all employees who manage themselves directly or indirectly in a company hierarchy stored in a table employees(id, name, manager_id). Why is a recursive CTE needed here?

hard

A. Because recursive CTEs store data permanently for future use

B. Because recursive CTEs automatically create indexes for faster queries

C. Because recursive CTEs can repeatedly join the table to find all levels of management

D. Because recursive CTEs replace the need for GROUP BY clauses

Solution

1. Step 1: Understand the problem of hierarchical queries

   Finding all managers up the chain requires repeatedly joining the table to itself for multiple levels.

2. Step 2: Recognize recursive CTE capability

   Recursive CTEs allow a query to call itself to traverse hierarchical data until all levels are found.

3. Final Answer:

   Because recursive CTEs can repeatedly join the table to find all levels of management -> Option C

4. Quick Check:

   Recursive CTEs handle hierarchical recursion = C [OK]

Hint: Use recursive CTEs for multi-level hierarchical queries [OK]

Common Mistakes:

• Thinking recursive CTEs create indexes
• Confusing recursive CTEs with permanent storage
• Assuming recursive CTEs replace GROUP BY