CTE vs subquery vs view decision in SQL - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a CTE named recent_orders.

SQL

WITH [1] AS (SELECT * FROM orders WHERE order_date > '2024-01-01') SELECT * FROM recent_orders;

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

Arecent_orders

Borders_recent

Corders_cte

Drecent

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different name in the CTE definition and the SELECT statement.

Using a name that is not descriptive or confusing.

The CTE name must match the name used after WITH and in the main query. 'recent_orders' is the correct name here.

2fill in blank

medium

Complete the code to write a subquery that selects customers with orders over 100.

SQL

SELECT customer_id FROM customers WHERE customer_id IN (SELECT [1] FROM orders WHERE amount > 100);

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

Aamount

Border_id

Ccustomer_id

Dorder_date

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting order_id instead of customer_id in the subquery.

Selecting amount or order_date which do not match the outer query condition.

The subquery must select customer_id to match the outer query's WHERE condition.

3fill in blank

hard

Fix the error in the view creation statement by filling the blank.

SQL

CREATE VIEW [1] AS SELECT product_id, SUM(quantity) AS total_qty FROM sales GROUP BY product_id;

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

Asales_summary

Bsummary_sales

Csales_total

Dproduct_sales

Attempts:

3 left

💡 Hint

Common Mistakes

Using a name that does not reflect the data.

Using reserved keywords or invalid characters in the view name.

The view name should be descriptive and consistent. 'sales_summary' clearly describes the content.

4fill in blank

hard

Fill both blanks to write a query using a CTE and filter results.

SQL

WITH high_sales AS (SELECT customer_id, SUM(amount) AS total FROM orders GROUP BY customer_id) SELECT * FROM high_sales WHERE total [1] [2];

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

A>

B1000

C<

D500

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' to filter high sales.

Using a low threshold that does not match the intent.

The query filters customers with total sales greater than 1000.

5fill in blank

hard

Fill all three blanks to create a view that shows average order amount per customer with a filter.

SQL

CREATE VIEW [1] AS SELECT customer_id, AVG(amount) AS avg_amount FROM orders GROUP BY [2] HAVING AVG(amount) [3] 50;

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

Acustomer_avg_order

Bcustomer_id

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong grouping column.

Using '<' instead of '>' in HAVING clause.

Choosing a view name that is unclear.

The view name is 'customer_avg_order', grouping is by 'customer_id', and the HAVING clause filters averages greater than 50.

Practice

1. Which SQL feature is best used to temporarily break down a complex query into simpler parts for easier reading and maintenance?

easy

A. Common Table Expression (CTE)

B. View

C. Subquery

D. Index

Solution

1. Step 1: Understand the purpose of CTEs

   CTEs allow you to define temporary named result sets that can be referenced within a query, making complex queries easier to read and maintain.

2. Step 2: Compare with other options

   Views are permanent saved queries, subqueries are nested queries but can be harder to read, and indexes are for performance, not query structure.

3. Final Answer:

   Common Table Expression (CTE) -> Option A

4. Quick Check:

   Temporary query breakdown = CTE [OK]

Hint: Temporary query parts? Use CTEs [OK]

Common Mistakes:

• Confusing views as temporary structures
• Thinking subqueries improve readability as much as CTEs
• Assuming indexes help with query structure

2. Which of the following is the correct syntax to define a Common Table Expression (CTE) in SQL?

easy

A. CREATE VIEW cte_name AS SELECT * FROM table_name;

B. WITH cte_name AS (SELECT * FROM table_name) SELECT * FROM cte_name;

C. SELECT * FROM (SELECT * FROM table_name) AS cte_name;

D. DEFINE cte_name SELECT * FROM table_name;

Solution

1. Step 1: Recall CTE syntax

   CTEs start with the keyword WITH, followed by the CTE name, AS, and a query in parentheses.

2. Step 2: Check each option

   WITH cte_name AS (SELECT * FROM table_name) SELECT * FROM cte_name; matches the correct syntax. CREATE VIEW cte_name AS SELECT * FROM table_name; is for views, C is a subquery, and D is invalid syntax.

3. Final Answer:

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

4. Quick Check:

   CTE syntax starts with WITH [OK]

Hint: CTE always starts with WITH keyword [OK]

Common Mistakes:

• Using CREATE VIEW syntax for CTE
• Confusing subquery parentheses with CTE
• Using undefined keywords like DEFINE

3. Given the following SQL code, what will be the output?

WITH SalesCTE AS (
  SELECT ProductID, SUM(Quantity) AS TotalSold
  FROM Sales
  GROUP BY ProductID
)
SELECT * FROM SalesCTE WHERE TotalSold > 100;

medium

A. Empty result set because SalesCTE is not materialized

B. All products with total quantity sold less than 100

C. Syntax error due to missing GROUP BY

D. All products with total quantity sold greater than 100

Solution

1. Step 1: Analyze the CTE query

   The CTE calculates total quantity sold per product by grouping Sales data by ProductID and summing Quantity.

2. Step 2: Understand the main query filter

   The main query selects rows from SalesCTE where TotalSold is greater than 100, so it returns products with sales over 100 units.

3. Final Answer:

   All products with total quantity sold greater than 100 -> Option D

4. Quick Check:

   CTE filters totals > 100 = correct output [OK]

Hint: CTE output filters rows after aggregation [OK]

Common Mistakes:

• Thinking CTE is not executed or materialized
• Confusing GROUP BY usage
• Assuming syntax error without checking carefully

4. You wrote this SQL query but it throws an error:

SELECT * FROM (
  SELECT id, name FROM users
) AS user_sub WHERE user_sub.age > 30;

What is the most likely cause of the error?

medium

A. The subquery alias 'user_sub' is missing

B. The WHERE clause should be inside the subquery

C. The subquery does not include the 'age' column needed in WHERE

D. Subqueries cannot be used in FROM clause

Solution

1. Step 1: Check subquery columns

   The subquery selects only 'id' and 'name' columns, but the outer query filters on 'age' which is not selected.

2. Step 2: Understand SQL column visibility

   Since 'age' is not in the subquery result, the outer WHERE clause referencing 'age' causes an error.

3. Final Answer:

   The subquery does not include the 'age' column needed in WHERE -> Option C

4. Quick Check:

   Missing column in subquery causes error [OK]

Hint: Ensure subquery selects all columns used outside [OK]

Common Mistakes:

• Forgetting to alias subquery
• Thinking subqueries can't be in FROM
• Moving WHERE inside subquery unnecessarily

5. You want to reuse a complex query frequently in different reports without rewriting it each time. Which SQL feature should you use and why?

hard

A. Create a View because it saves the query in the database for easy reuse.

B. Use a CTE because it is permanent and can be called anytime.

C. Use a subquery because it is stored and optimized by the database.

D. Create an index because it stores the query results for reuse.

Solution

1. Step 1: Understand the need for reuse

   Reusing a complex query multiple times means you want a saved, reusable object in the database.

2. Step 2: Compare features for reuse

   Views are saved queries stored in the database and can be used repeatedly in different reports. CTEs are temporary and exist only during query execution. Subqueries are inline and not stored. Indexes improve performance but do not store queries.

3. Final Answer:

   Create a View because it saves the query in the database for easy reuse. -> Option A

4. Quick Check:

   Persistent reusable query = View [OK]

Hint: Persistent reusable query? Use a View [OK]

Common Mistakes:

• Confusing CTEs as permanent
• Thinking subqueries are stored
• Misunderstanding indexes as query storage