Dropping and altering views in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What happens after dropping a view?

Consider a view named employee_summary created on a table employees. After executing the command DROP VIEW employee_summary;, what will be the result of querying SELECT * FROM employee_summary;?

SQL

DROP VIEW employee_summary;
SELECT * FROM employee_summary;

A The query raises an error because the view no longer exists.

B The query returns an empty result set with the same columns as the view.

C The query returns all rows from the original employees table.

D The query returns the last cached result of the view before it was dropped.

💡 Hint

Think about what happens to a view after it is dropped and whether it still exists for querying.

When a view is dropped, it is removed from the database. Any query referencing that view will raise an error because the view no longer exists.

📝 Syntax

intermediate

Identify the correct syntax to alter a view

You want to change the definition of an existing view named customer_orders to include only orders from the last year. Which of the following SQL commands correctly updates the view?

A CREATE OR REPLACE VIEW customer_orders AS SELECT * FROM orders WHERE order_date >= CURRENT_DATE - INTERVAL '1 year';

B UPDATE VIEW customer_orders SET definition = 'SELECT * FROM orders WHERE order_date >= CURRENT_DATE - INTERVAL ''1 year''';

C ALTER VIEW customer_orders AS SELECT * FROM orders WHERE order_date >= CURRENT_DATE - INTERVAL '1 year';

D MODIFY VIEW customer_orders AS SELECT * FROM orders WHERE order_date >= CURRENT_DATE - INTERVAL '1 year';

💡 Hint

Think about how to redefine a view without dropping it first.

Standard SQL does not support ALTER VIEW to change the query. Instead, CREATE OR REPLACE VIEW is used to redefine the view.

🧠 Conceptual

advanced

Effect of dropping a view on dependent objects

If a view sales_summary is dropped, which of the following statements about objects depending on this view is true?

A All dependent views and stored procedures will automatically update to remove references to the dropped view.

B Dropping the view will fail if there are dependent objects referencing it, unless those objects are dropped first.

C Dependent objects will continue to work normally because they use cached data from the dropped view.

D Dropping the view will cascade and automatically drop all dependent objects referencing it.

💡 Hint

Consider how databases handle dependencies when dropping objects.

Most databases prevent dropping a view if other objects depend on it, unless those dependent objects are dropped first or the drop is done with CASCADE.

🔧 Debug

advanced

Why does this ALTER VIEW command fail?

Given the command below, why does it fail in most SQL databases?

ALTER VIEW product_list AS SELECT id, name FROM products WHERE active = TRUE;

SQL

ALTER VIEW product_list AS SELECT id, name FROM products WHERE active = TRUE;

A The SELECT statement must include all columns from the base table.

B The WHERE clause is not allowed in view definitions.

C The view name must be enclosed in quotes to alter it.

D ALTER VIEW does not support redefining the SELECT statement; CREATE OR REPLACE VIEW should be used instead.

💡 Hint

Think about the standard way to change a view's query.

ALTER VIEW is not supported for changing the query in many SQL dialects. Instead, CREATE OR REPLACE VIEW is used.

❓ optimization

expert

Optimizing view updates with minimal downtime

You need to update a large view monthly_report used by many users. Dropping and recreating the view causes errors for users querying it during the update. Which approach minimizes downtime and errors?

A Rename the old view, create a new view with the original name, then drop the old view after users switch.

B Use <code>DROP VIEW monthly_report;</code> then <code>CREATE VIEW monthly_report AS ...;</code> during off-hours only.

C Use <code>CREATE OR REPLACE VIEW monthly_report AS ...;</code> to atomically update the view definition without dropping it first.

D Create a new view with a different name and ask users to switch manually to the new view.

💡 Hint

Consider atomic operations that do not remove the view before recreating it.

CREATE OR REPLACE VIEW updates the view atomically, so users querying the view do not experience errors or downtime.

Practice

1. What does the SQL command DROP VIEW view_name; do?

easy

A. It updates the data inside the view view_name.

B. It renames the view view_name to another name.

C. It creates a new view called view_name.

D. It deletes the view named view_name from the database.

Solution

1. Step 1: Understand the purpose of DROP VIEW

   The command DROP VIEW is used to remove a view from the database completely.

2. Step 2: Analyze the command effect

   Using DROP VIEW view_name; deletes the view named view_name, so it no longer exists.

3. Final Answer:

   It deletes the view named view_name from the database. -> Option D

4. Quick Check:

   DROP VIEW removes a view [OK]

Hint: DROP VIEW deletes a view completely from the database [OK]

Common Mistakes:

• Thinking DROP VIEW updates data inside the view
• Confusing DROP VIEW with CREATE VIEW
• Assuming DROP VIEW renames the view

2. Which of the following is the correct syntax to change the query inside an existing view named employee_view?

easy

A. MODIFY VIEW employee_view SELECT * FROM employees;

B. UPDATE VIEW employee_view SET query = 'SELECT * FROM employees';

C. ALTER VIEW employee_view AS SELECT * FROM employees WHERE active = 1;

D. CHANGE VIEW employee_view TO SELECT * FROM employees;

Solution

1. Step 1: Recall ALTER VIEW syntax

   The correct syntax to change a view's query is ALTER VIEW view_name AS SELECT ....

2. Step 2: Check each option

   ALTER VIEW employee_view AS SELECT * FROM employees WHERE active = 1; uses the correct syntax. Options B, C, and D use invalid or non-existent commands.

3. Final Answer:

   ALTER VIEW employee_view AS SELECT * FROM employees WHERE active = 1; -> Option C

4. Quick Check:

   ALTER VIEW uses AS SELECT [OK]

Hint: Use ALTER VIEW view_name AS SELECT ... to update a view [OK]

Common Mistakes:

• Using UPDATE or MODIFY instead of ALTER
• Omitting AS keyword
• Trying to rename view with ALTER VIEW

3. Given a view active_customers defined as SELECT * FROM customers WHERE status = 'active', what will be the result after running ALTER VIEW active_customers AS SELECT * FROM customers WHERE status = 'inactive'; and then querying SELECT * FROM active_customers;?

medium

A. It will return all customers with status 'inactive'.

B. It will cause a syntax error.

C. It will return all customers regardless of status.

D. It will return all customers with status 'active'.

Solution

1. Step 1: Understand ALTER VIEW effect

   The ALTER VIEW command changes the query inside the view to the new SELECT statement.

2. Step 2: Analyze the new query

   The view now selects customers where status = 'inactive'. So querying the view returns inactive customers.

3. Final Answer:

   It will return all customers with status 'inactive'. -> Option A

4. Quick Check:

   ALTER VIEW changes the view's query [OK]

Hint: ALTER VIEW changes the query; output matches new SELECT [OK]

Common Mistakes:

• Assuming view still returns old data after ALTER
• Thinking ALTER VIEW causes errors if view exists
• Confusing view data with table data

4. You try to run ALTER VIEW sales_view AS SELECT * FROM sales WHERE amount > 1000; but get an error. What is the most likely cause?

medium

A. The view sales_view does not exist.

B. The SELECT query inside ALTER VIEW is missing a semicolon.

C. You cannot use WHERE clauses inside ALTER VIEW.

D. ALTER VIEW requires DROP VIEW before use.

Solution

1. Step 1: Check ALTER VIEW prerequisites

   ALTER VIEW requires the view to already exist to modify it.

2. Step 2: Identify common error cause

   If the view sales_view does not exist, ALTER VIEW will fail with an error.

3. Final Answer:

   The view sales_view does not exist. -> Option A

4. Quick Check:

   ALTER VIEW fails if view missing [OK]

Hint: Ensure view exists before ALTER VIEW [OK]

Common Mistakes:

• Thinking WHERE clause is invalid in ALTER VIEW
• Assuming semicolon inside ALTER VIEW causes error
• Believing DROP VIEW is needed before ALTER VIEW

5. You have a view product_summary showing product names and total sales. You want to update it to include only products with sales over 500. Which sequence of commands correctly updates the view without losing it?

hard

A. DROP VIEW product_summary; CREATE VIEW product_summary AS SELECT name, SUM(sales) FROM products GROUP BY name HAVING SUM(sales) > 500;

B. ALTER VIEW product_summary AS SELECT name, SUM(sales) FROM products GROUP BY name HAVING SUM(sales) > 500;

C. UPDATE VIEW product_summary SET query = 'SELECT name, SUM(sales) FROM products GROUP BY name HAVING SUM(sales) > 500;';

D. REPLACE VIEW product_summary AS SELECT name, SUM(sales) FROM products GROUP BY name HAVING SUM(sales) > 500;

Solution

1. Step 1: Understand how to update a view's query

   ALTER VIEW updates the query inside an existing view without dropping it.

2. Step 2: Check each option's correctness

   ALTER VIEW product_summary AS SELECT name, SUM(sales) FROM products GROUP BY name HAVING SUM(sales) > 500; uses ALTER VIEW with the correct query. DROP VIEW product_summary; CREATE VIEW product_summary AS SELECT name, SUM(sales) FROM products GROUP BY name HAVING SUM(sales) > 500; drops and recreates the view, which is not needed. Options B and D use invalid commands.

3. Final Answer:

   ALTER VIEW product_summary AS SELECT name, SUM(sales) FROM products GROUP BY name HAVING SUM(sales) > 500; -> Option B

4. Quick Check:

   Use ALTER VIEW to update view query safely [OK]

Hint: Use ALTER VIEW to update query without dropping [OK]

Common Mistakes:

• Dropping view unnecessarily before updating
• Using UPDATE VIEW or REPLACE VIEW which are invalid
• Not including HAVING clause in the new query