What is a View in SQL?

What is a View in SQL?

SQL is a powerful language for manipulating data in relational databases. It allows you to create, read, update, and delete data using various commands and clauses. However, sometimes you may need to perform complex operations on data that involve multiple tables, conditions, or calculations. In such cases, writing SQL queries from scratch every time can be tedious and error-prone. Moreover, you may want to restrict the access to some data for security or privacy reasons. This is where SQL views come in handy.

What is a SQL View?

A SQL view is a virtual table that is based on the result of an SQL query. A view does not store any data on the disk; it only contains the definition of the query that generates the data. You can think of a view as a saved query that you can use as a table.

A view has several advantages over a regular table:

• It simplifies complex queries by hiding the details of the underlying tables and logic.
• It provides a consistent and convenient way to access data that may change frequently or come from different sources.
• It enhances security and privacy by limiting the exposure of sensitive data or allowing different levels of access for different users.
• It improves performance and efficiency by caching the results of the query and avoiding redundant computations.

How to Create a View in SQL?

You can create a view in SQL using the CREATE VIEW statement. The syntax is as follows:

CREATE VIEW view_name AS
SELECT column1, column2, ...
FROM table_name
WHERE condition;

The view_name is the name of the view that you want to create. The SELECT statement defines the query that generates the data for the view. You can use any valid SQL expression in the SELECT statement, such as joins, aggregations, functions, aliases, etc.

For example, suppose you have a table called Customers that contains information about your customers, such as their name, country, phone number, email address, etc. You can create a view called Brazil_Customers that shows only the customers from Brazil and their contact details using the following query:

CREATE VIEW Brazil_Customers AS
SELECT CustomerName, Phone, Email
FROM Customers
WHERE Country = 'Brazil';

How to Use a View in SQL?

Once you have created a view, you can use it just like any other table in SQL. You can query it, join it with other tables or views, insert, update, or delete data from it, etc. However, some operations may have some limitations or restrictions depending on how the view is defined and what kind of data it contains.

To query a view, you can use the SELECT statement with the view name as the table name. For example, you can query the Brazil_Customers view that we created earlier using the following query:

SELECT * FROM Brazil_Customers;

This will return all the rows and columns from the view, which are generated by executing the underlying query on the Customers table.

You can also apply filters, order by clauses, group by clauses, etc. to the view as you would do with a regular table. For example, you can query the Brazil_Customers view to show only the customers whose name starts with ‘A’ and order them by their email address using the following query:

SELECT * FROM Brazil_Customers
WHERE CustomerName LIKE 'A%'
ORDER BY Email;

To join a view with another table or view, you can use the JOIN clause with the view name as one of the table names. For example, suppose you have another table called Orders that contains information about the orders placed by your customers, such as their order ID, customer ID, order date, total amount, etc. You can join the Brazil_Customers view with the Orders table to show the total amount of orders placed by each customer from Brazil using the following query:

SELECT b.CustomerName, SUM(o.TotalAmount) AS TotalOrders
FROM Brazil_Customers b
JOIN Orders o ON b.CustomerID = o.CustomerID
GROUP BY b.CustomerName;

This will return a result set with two columns: CustomerName and TotalOrders.

To insert, update, or delete data from a view, you need to make sure that the view is updatable. A view is updatable if it meets certain conditions¹, such as:

• The view is based on one and only one table.
• The view includes all the columns from that table that are part of its primary key or have a NOT NULL constraint.
• The view does not contain any aggregate functions, DISTINCT clause, GROUP BY clause, HAVING clause, UNION clause, subqueries, etc.

If a view is updatable, you can use the INSERT INTO statement to insert new rows into it², which will also insert them into the underlying table. For example,

INSERT INTO Brazil_Customers (CustomerID,CustomerName,Country)
VALUES (101,'Ana','Brazil');

This will insert a new row into the Brazil_Customers view and the Customers table with the values specified.

You can also use the UPDATE statement to update existing rows in the view³, which will also update them in the underlying table. For example,

UPDATE Brazil_Customers
SET Phone = '+55 11 1234-5678'
WHERE CustomerName = 'Ana';

This will update the phone number of the customer named ‘Ana’ in the Brazil_Customers view and the Customers table.

Similarly, you can use the DELETE statement to delete rows from the view⁴, which will also delete them from the underlying table. For example,

DELETE FROM Brazil_Customers
WHERE CustomerName = 'Ana';

This will delete the row of the customer named ‘Ana’ from the Brazil_Customers view and the Customers table.

How to Modify or Drop a View in SQL?

If you want to change the definition of a view, you can use the CREATE OR REPLACE VIEW statement. This will replace the existing view with a new one that has the same name but a different query. The syntax is as follows:

CREATE OR REPLACE VIEW view_name AS
SELECT column1, column2, ...
FROM table_name
WHERE condition;

For example, if you want to add the city column to the Brazil_Customers view, you can use the following query:

CREATE OR REPLACE VIEW Brazil_Customers AS
SELECT CustomerName, City, Phone, Email
FROM Customers
WHERE Country = 'Brazil';

This will replace the existing Brazil_Customers view with a new one that includes the city column.

If you want to delete a view, you can use the DROP VIEW statement. This will remove the view and its definition from the database. The syntax is as follows:

DROP VIEW view_name;

For example, if you want to delete the Brazil_Customers view, you can use the following query:

DROP VIEW Brazil_Customers;

This will drop the Brazil_Customers view from the database.

Real-World Examples of SQL Views

SQL views are widely used in real-world applications for various purposes. Here are some examples of how SQL views can be useful in different scenarios:

• Data analysis and reporting: You can create views to perform complex calculations or transformations on data and present them in a simple and consistent way. For example, you can create a view that shows the monthly sales report for each product category by joining multiple tables and applying aggregate functions.
• Data abstraction and encapsulation: You can create views to hide the complexity and details of the underlying data structures and provide a higher level of abstraction for users or applications. For example, you can create a view that shows only the relevant information for a user profile by joining multiple tables and filtering out unnecessary columns.
• Data security and access control: You can create views to restrict or grant access to certain data for different users or roles. For example, you can create a view that shows only the public information for a customer by excluding any sensitive or personal data.
• Data validation and integrity: You can create views to enforce certain rules or constraints on data and prevent invalid or inconsistent data from being entered or modified. For example, you can create a view that checks if an order has a valid customer ID and order date before inserting it into the database.

Conclusion

In this article, we have learned what SQL views are, how to create, use, modify, and drop them, and what are some of their advantages and applications. SQL views are a powerful feature that allows you to create virtual tables based on queries and use them as regular tables. SQL views can simplify complex queries, provide consistent and convenient access to data, enhance security and privacy, improve performance and efficiency, and more. SQL views are widely used in real-world applications for data analysis, reporting, abstraction, encapsulation, security, access control, validation, integrity, and more.

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