CREATE VIEW syntax in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a VIEW in a database?

A VIEW is a virtual table that shows data from one or more tables. It does not store data itself but displays data based on a query.

beginner

Basic syntax to create a VIEW in PostgreSQL?

Use:
CREATE VIEW view_name AS SELECT column1, column2 FROM table_name;
This creates a virtual table named view_name based on the SELECT query.

intermediate

Can you update data through a VIEW?

Usually, you cannot update data through a VIEW unless it is a simple view and the database supports updatable views. Complex views often are read-only.

intermediate

What happens if you try to create a VIEW that already exists without using IF NOT EXISTS?

PostgreSQL will return an error saying the view already exists. To avoid this, use CREATE OR REPLACE VIEW to update the existing view or drop it first.

intermediate

Explain the purpose of the CREATE OR REPLACE VIEW statement.

It creates a new view if it doesn't exist or replaces the definition of an existing view without dropping it. This helps update views safely.

Which statement correctly creates a view named 'employee_view' showing 'name' and 'salary' from 'employees' table?

A CREATE VIEW employee_view AS SELECT name, salary FROM employees;

B CREATE TABLE employee_view AS SELECT name, salary FROM employees;

C SELECT name, salary INTO employee_view FROM employees;

D CREATE employee_view VIEW AS SELECT name, salary FROM employees;

Option A uses the correct syntax to create a view in PostgreSQL.

What does a VIEW store in the database?

A Only the query definition

B Actual data rows

C Indexes for fast access

D Temporary data copies

A VIEW stores only the query definition, not the actual data.

How can you modify an existing view without dropping it first?

A CREATE VIEW view_name AS ...

B ALTER VIEW view_name AS ...

C CREATE OR REPLACE VIEW view_name AS ...

D DROP VIEW view_name; CREATE VIEW view_name AS ...

CREATE OR REPLACE VIEW updates the view definition without dropping it.

If you create a view with the same name as an existing one without OR REPLACE, what happens?

A The old view is replaced automatically

B Both views exist with the same name

C The new view is ignored

D An error is raised

PostgreSQL raises an error if a view with the same name exists.

Which of these is NOT a benefit of using views?

A Simplify complex queries

B Store large amounts of data physically

C Improve data security by restricting columns

D Provide a consistent interface to data

Views do not store data physically; they only show data from tables.

Describe how to create a simple view in PostgreSQL and explain what it does.

Think about how a view acts like a saved SELECT query.

Explain the difference between CREATE VIEW and CREATE OR REPLACE VIEW.

Consider what happens if the view already exists.

Practice

1. What is the main purpose of using CREATE VIEW in PostgreSQL?

easy

A. To delete rows from a table

B. To save a SELECT query as a virtual table for easy reuse

C. To permanently store data in a new table

D. To create an index on a table column

Solution

1. Step 1: Understand what a view is

   A view is a virtual table that stores a SELECT query, not actual data.

2. Step 2: Identify the purpose of CREATE VIEW

   CREATE VIEW saves the SELECT query so you can reuse it easily without rewriting it.

3. Final Answer:

   To save a SELECT query as a virtual table for easy reuse -> Option B

4. Quick Check:

   CREATE VIEW = virtual table for queries [OK]

Hint: Views save SELECT queries as virtual tables [OK]

Common Mistakes:

• Thinking views store data permanently
• Confusing views with tables
• Using CREATE VIEW to delete data

2. Which of the following is the correct syntax to create a view named active_users showing users with status = 'active'?

easy

A. CREATE VIEW active_users AS SELECT * FROM users WHERE status = 'active';

B. CREATE active_users VIEW AS SELECT * FROM users WHERE status = 'active';

C. VIEW CREATE active_users AS SELECT * FROM users WHERE status = 'active';

D. CREATE VIEW active_users SELECT * FROM users WHERE status = 'active';

Solution

1. Step 1: Recall the correct CREATE VIEW syntax

   The syntax is: CREATE VIEW view_name AS SELECT ...

2. Step 2: Match the syntax with options

   CREATE VIEW active_users AS SELECT * FROM users WHERE status = 'active'; matches the correct syntax exactly.

3. Final Answer:

   CREATE VIEW active_users AS SELECT * FROM users WHERE status = 'active'; -> Option A

4. Quick Check:

   CREATE VIEW view_name AS SELECT ... [OK]

Hint: Syntax is CREATE VIEW name AS SELECT ... [OK]

Common Mistakes:

• Swapping CREATE and VIEW keywords
• Omitting AS keyword
• Missing semicolon at the end

3. Given the table orders with columns order_id, customer_id, and amount, what will be the result of this view?

CREATE VIEW big_orders AS SELECT order_id, amount FROM orders WHERE amount > 100;

What rows will SELECT * FROM big_orders; return?

medium

A. No rows, syntax error in view

B. All orders with amount less than or equal to 100

C. All orders regardless of amount

D. All orders with amount greater than 100

Solution

1. Step 1: Analyze the SELECT query inside the view

   The view selects order_id and amount from orders where amount is greater than 100.

2. Step 2: Understand what SELECT * FROM big_orders returns

   It returns all rows from the view, which are orders with amount > 100.

3. Final Answer:

   All orders with amount greater than 100 -> Option D

4. Quick Check:

   View filters orders with amount > 100 [OK]

Hint: View returns rows matching its SELECT query condition [OK]

Common Mistakes:

• Confusing greater than with less than
• Expecting all columns instead of selected ones
• Assuming syntax error without checking carefully

4. Identify the error in this CREATE VIEW statement:

CREATE VIEW recent_customers AS SELECT id, name FROM customers WHERE;

medium

A. Missing condition after WHERE keyword

B. Missing AS keyword before SELECT

C. View name cannot be recent_customers

D. SELECT must include all columns

Solution

1. Step 1: Check the syntax after WHERE

   The WHERE clause is incomplete; it must have a condition after WHERE.

2. Step 2: Verify other parts of the statement

   AS keyword is present, view name is valid, and SELECT columns are allowed to be partial.

3. Final Answer:

   Missing condition after WHERE keyword -> Option A

4. Quick Check:

   WHERE must have a condition [OK]

Hint: WHERE clause must have a condition [OK]

Common Mistakes:

• Leaving WHERE empty
• Forgetting AS keyword
• Thinking view names have restrictions

5. You want to create a view customer_orders that shows each customer's name and the total amount they spent. Which of these is the correct way to create this view assuming tables customers(id, name) and orders(customer_id, amount)?

hard

A. CREATE VIEW customer_orders AS SELECT c.name, o.amount FROM customers c, orders o WHERE c.id = o.customer_id;

B. CREATE VIEW customer_orders AS SELECT name, amount FROM customers JOIN orders ON id = customer_id;

C. CREATE VIEW customer_orders AS SELECT c.name, SUM(o.amount) FROM customers c JOIN orders o ON c.id = o.customer_id GROUP BY c.name;

D. CREATE VIEW customer_orders AS SELECT name, SUM(amount) FROM orders GROUP BY name;

Solution

1. Step 1: Understand the requirement

   We need each customer's name and total amount spent, so we must join customers and orders and sum amounts grouped by customer name.

2. Step 2: Check each option

   CREATE VIEW customer_orders AS SELECT c.name, SUM(o.amount) FROM customers c JOIN orders o ON c.id = o.customer_id GROUP BY c.name; correctly joins tables, sums amounts, and groups by customer name. Others miss grouping or join conditions.

3. Final Answer:

   CREATE VIEW customer_orders AS SELECT c.name, SUM(o.amount) FROM customers c JOIN orders o ON c.id = o.customer_id GROUP BY c.name; -> Option C

4. Quick Check:

   Join + SUM + GROUP BY = correct view [OK]

Hint: Use JOIN with GROUP BY and SUM for aggregation views [OK]

Common Mistakes:

• Missing GROUP BY when using SUM
• Not joining tables properly
• Selecting columns not in GROUP BY