View as a saved query mental model in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a SQL view?

A SQL view is like a saved query. It shows data from one or more tables but does not store data itself. It acts like a virtual table.

beginner

How does a view help simplify complex queries?

A view saves a complex query so you can use it easily later. You just select from the view instead of writing the full query again.

intermediate

Can you update data through a view?

Sometimes yes, if the view is simple and directly maps to one table. But if the view joins many tables or uses aggregation, updates usually are not allowed.

beginner

What happens when you query a view?

The database runs the saved query behind the view and shows the results as if it was a table.

beginner

Why use views instead of tables?

Views help keep data safe by hiding columns or rows. They also make queries easier and keep your database organized.

What does a SQL view store?

A The result of a query as data

B A saved query definition

C A backup of a table

D Indexes for faster search

A view stores the query definition, not the actual data.

Which of these is true about views?

A Views can simplify complex queries

B Views are the same as tables

C Views cannot be used in SELECT statements

D Views always store data physically

Views save complex queries so you can use them easily.

Can you update data through any view?

A Yes, all views allow updates

B No, views never allow updates

C Only simple views that map to one table allow updates

D Only views with joins allow updates

Updates through views are allowed only if the view is simple and directly linked to one table.

What is the main benefit of using a view?

A To speed up data entry

B To store large amounts of data

C To create backups of tables

D To hide complex queries and simplify access

Views hide complex queries and make it easier to access data.

When you select from a view, what happens?

A The database runs the saved query behind the view

B The view data is copied to a new table

C The view data is deleted

D Nothing happens, views cannot be selected

Selecting from a view runs the saved query and shows the results.

Explain what a SQL view is and how it works as a saved query.

Think of a view as a shortcut to a query.

Describe the benefits and limitations of using views in a database.

Consider both what views help with and what they cannot do.

Practice

1. What is a SQL view best described as?

easy

A. A backup copy of a database

B. A physical table storing data permanently

C. A saved query that acts like a virtual table

D. A user account with special permissions

Solution

1. Step 1: Understand what a view is

   A view is not a physical table but a saved SQL query that can be treated like a table.

2. Step 2: Compare options to definition

   A saved query that acts like a virtual table matches the definition exactly. The other options describe different database concepts like backups, physical tables, or user permissions.

3. Final Answer:

   A saved query that acts like a virtual table -> Option C

4. Quick Check:

   View = saved query acting like table [OK]

Hint: Remember: Views are saved queries, not real tables [OK]

Common Mistakes:

• Thinking views store data physically
• Confusing views with backups
• Assuming views are user accounts

2. Which of the following is the correct syntax to create a view named EmployeeView showing all columns from Employees table?

easy

A. VIEW CREATE EmployeeView SELECT * FROM Employees;

B. CREATE TABLE EmployeeView AS SELECT * FROM Employees;

C. SELECT * INTO EmployeeView FROM Employees;

D. CREATE VIEW EmployeeView AS SELECT * FROM Employees;

Solution

1. Step 1: Recall the syntax for creating a view

   The correct syntax starts with CREATE VIEW, followed by the view name, AS, then the SELECT query.

2. Step 2: Check each option

   CREATE VIEW EmployeeView AS SELECT * FROM Employees; matches the correct syntax. CREATE TABLE EmployeeView AS SELECT * FROM Employees; creates a table, not a view. SELECT * INTO EmployeeView FROM Employees; is for SELECT INTO which creates a table. VIEW CREATE EmployeeView SELECT * FROM Employees; is invalid syntax.

3. Final Answer:

   CREATE VIEW EmployeeView AS SELECT * FROM Employees; -> Option D

4. Quick Check:

   CREATE VIEW ... AS SELECT ... [OK]

Hint: Use CREATE VIEW ... AS SELECT ... to define views [OK]

Common Mistakes:

• Using CREATE TABLE instead of CREATE VIEW
• Confusing SELECT INTO with view creation
• Incorrect keyword order in syntax

3. Given the table Products with columns id, name, and price, and the view created as:
CREATE VIEW CheapProducts AS SELECT id, name FROM Products WHERE price < 50;
What will the query SELECT * FROM CheapProducts; return if Products contains:

id | name    | price
1  | Pen     | 10
2  | Notebook| 60
3  | Eraser  | 30

medium

A. Rows with id 1, 2, and 3 showing all columns

B. Rows with id 1 and 3 showing id and name columns

C. Rows with id 2 only showing id and name columns

D. No rows returned because price is not selected

Solution

1. Step 1: Understand the view definition

   The view selects id and name from Products where price is less than 50.

2. Step 2: Apply the filter to the data

   Products with price less than 50 are id 1 (price 10) and id 3 (price 30). The view returns only id and name columns.

3. Final Answer:

   Rows with id 1 and 3 showing id and name columns -> Option B

4. Quick Check:

   View filters price < 50 and selects id, name [OK]

Hint: View returns filtered columns and rows as defined [OK]

Common Mistakes:

• Expecting all columns in view output
• Including rows that don't meet WHERE condition
• Thinking view stores data separately

4. You created a view with:
CREATE VIEW ActiveUsers AS SELECT id, name FROM Users WHERE active = 1;
But running SELECT * FROM ActiveUsers; gives an error: ERROR: relation "activeusers" does not exist
What is the most likely cause?

medium

A. The view was not created successfully or was dropped

B. The SELECT query inside the view has syntax errors

C. The Users table does not have an 'active' column

D. You must refresh the view before selecting from it

Solution

1. Step 1: Analyze the error message

   The error says the relation (view) "activeusers" does not exist, meaning the view is missing.

2. Step 2: Consider causes for missing view

   This usually means the view was never created or was dropped. Syntax errors or missing columns cause errors during creation, not this runtime error. Views do not require refreshing.

3. Final Answer:

   The view was not created successfully or was dropped -> Option A

4. Quick Check:

   Missing view = creation failed or dropped [OK]

Hint: Check if view exists before querying it [OK]

Common Mistakes:

• Assuming syntax errors cause this runtime error
• Thinking views need refreshing like materialized views
• Ignoring case sensitivity or schema issues

5. You want to create a view RecentOrders that always shows orders placed in the last 7 days from the Orders table with columns order_id, customer_id, and order_date. Which SQL statement correctly creates this view?

hard

A. CREATE VIEW RecentOrders AS SELECT order_id, customer_id, order_date FROM Orders WHERE order_date > CURRENT_DATE - INTERVAL '7 days';

B. CREATE VIEW RecentOrders AS SELECT order_id, customer_id FROM Orders WHERE order_date > CURRENT_DATE - INTERVAL '7 days';

C. CREATE VIEW RecentOrders AS SELECT * FROM Orders WHERE order_date > DATEADD(day, -7, GETDATE());

D. CREATE VIEW RecentOrders AS SELECT order_id, customer_id FROM Orders WHERE order_date > SYSDATE - 7;

Solution

1. Step 1: Identify needed columns and date filter

   The view should include order_id, customer_id, and order_date columns and filter orders from last 7 days.

2. Step 2: Check each option's correctness

   CREATE VIEW RecentOrders AS SELECT order_id, customer_id FROM Orders WHERE order_date > CURRENT_DATE - INTERVAL '7 days'; misses order_date column, so it won't show the date. CREATE VIEW RecentOrders AS SELECT order_id, customer_id, order_date FROM Orders WHERE order_date > CURRENT_DATE - INTERVAL '7 days'; includes all needed columns and uses correct standard SQL interval syntax. CREATE VIEW RecentOrders AS SELECT * FROM Orders WHERE order_date > DATEADD(day, -7, GETDATE()); uses SQL Server syntax (DATEADD, GETDATE) which may not be standard. CREATE VIEW RecentOrders AS SELECT order_id, customer_id FROM Orders WHERE order_date > SYSDATE - 7; uses SYSDATE which is Oracle-specific and may not work in standard SQL.

3. Final Answer:

   CREATE VIEW RecentOrders AS SELECT order_id, customer_id, order_date FROM Orders WHERE order_date > CURRENT_DATE - INTERVAL '7 days'; -> Option A

4. Quick Check:

   Include needed columns and use standard interval syntax [OK]

Hint: Include all needed columns and use standard date intervals [OK]

Common Mistakes:

• Omitting important columns in view
• Using non-standard date functions
• Forgetting to filter by date correctly