Views in SQL
A view in SQL is a fictitious table that is constructed using the results of a SQL query, such as a SELECT statement. Although it has a set of columns and rows, unlike a table, it does not actually hold the data. Instead, it is a saved SQL query that you may use like a table by referencing it by name. When the view is accessed, a predetermined saved query is simply run.
Define the columns and choose the data that should be displayed in the view when establishing a view. In the same way that a table can be accessed by other SQL commands, the view is then saved in the database. Views can be used to organise data presentation for users, provide security by limiting access to specific columns or rows of data, connect data from several tables into a single view, aggregate data in a specific fashion, and simplify complex searches.
Since the view is essentially a query on the underlying tables, the data in the view changes whenever the data in the underlying tables does. Similar to tables, views can be changed, removed, or updated; but, depending on the particular database management system being used, there may be some limitations. As a result, the view always displays the most recent data from the underlying tables.
Once a view has been built, it may be queried using SQL SELECT statements just like a regular table and, depending on the user's permissions, can also be used for INSERT, UPDATE, and DELETE operations.
Views in SQL are used for a variety of reasons or purposes including:
Simplifying complex queries: Views can be used to simplify complex SQL queries by adding an abstraction layer that hides the complexity of the query's underlying structure. The query may become simpler to comprehend and manage as a result.
Enhancing security: Access to specific columns or rows of data can be restricted using views, which can improve security. Make a view, for instance, that only displays particular sensitive information to specific users or roles.
Creating virtual tables: Views can be made from one or more existing tables to create virtual tables. This can be helpful for building virtual tables that serve a certain function, like aggregating data or filtering data according to predetermined standards.
Providing a consistent interface: Even when the underlying tables change, views can still offer a consistent user experience for the data in a database. Applications that rely on the data in the database may be simpler to maintain as a result.
Simplifying data access: By generating pre-defined queries that can be accessed by numerous people, views can be utilized to streamline data access. This can lessen the amount of redundant code and make application maintenance simpler.
Restricting access to data: Views can be used to limit access to specific data rows or columns. Make a view, for instance, that just displays the names and salaries of employees while concealing any other personal information.
Aggregating data: Views can be used to aggregate data in a particular way. For example, create a view that shows the total sales for each region, or the average age of employees in each department.
Combining data from multiple tables: Data from different tables can be combined into one view using views. Working with complex data structures could become simpler as a result.
Providing a consistent interface: Regardless of the underlying table structure, views can offer a consistent interface to data. When utilizing numerous databases with various schemas or legacy systems, this can be helpful.
Basic steps for creating and using a view in SQL:-
Create the SQL query that defines the view: This SQL query will define the data that the view will display. It can include joins, filters, aggregates, and other SQL operations.
Use the CREATE VIEW statement to create the view: The CREATE VIEW statement is used to create a new view. This statement includes the name of the view, the SQL query that defines the view, and any other options or permissions that are needed.
Use the SELECT statement to query the view: Once the view is created, use the SELECT statement to query the view just like query a regular table. Use other SQL statements such as INSERT, UPDATE, and DELETE to modify the data in the view.
Modify or drop the view as needed: Modify the view by altering the SQL query that defines the view using the ALTER VIEW statement. Drop the view using the DROP VIEW statement when it is no longer needed.
Here's an example of how to create and use a simple view in SQL:
-- Create a view that shows the name and price of all products CREATE VIEW product_list AS SELECT product_name, product_price FROM products; -- Query the view to get the list of products SELECT * FROM product_list; -- Modify the view to show only products that are in stock ALTER VIEW product_list AS SELECT product_name, product_price FROM products WHERE in_stock = 1; -- Query the modified view to get the list of products in stock SELECT * FROM product_list; -- Drop the view when it is no longer needed DROP VIEW product_list;
First, in this example, a view called product_list was developed, which displays the names and prices of every product. Once you get the list of products, query the view. Use the ALTER VIEW statement to change the view so that it only displays items that are currently available. After getting the inventory list from the changed view using a query, use the DROP VIEW statement to remove the view. By building a virtual table based on the output of a SELECT command, views are employed in SQL. One or more tables may be included in the SELECT statement, along with any of the common SQL clauses like WHERE, GROUP BY, and ORDER BY.
To create a view in SQL, use the CREATE VIEW statement, followed by the name of the view and the SELECT statement that defines the view. Here's an example of a simple view that selects two columns from a table called "orders":
CREATE VIEW order_details AS SELECT order_id, order_date FROM orders;
Once created a view, query it just like querying a regular table.
For example, use a SELECT statement to retrieve data from the view.
SELECT order_id, order_date FROM order_details WHERE order_date > '2023-02-03';
This query would retrieve all the order IDs and order dates from the "order_details" view where the order date is greater than February 3rd, 2023.
Perform other operations on a view, such as inserting, updating, and deleting data, as long as the view is updatable. However, there are some restrictions on which views can be updated and how they can be updated, so it's important to check the documentation for specific database system.
Examples of Views in SQL:
Here are some examples of views in SQL:
- Simple view: This is a view that selects one or more columns from a single table. For example:
CREATE VIEW employee_names AS SELECT first_name, last_name FROM employees;
This view would create a virtual table called "employee_names" that contains the first name and last name columns from the "employees" table.
- Joined view: This is a view that joins two or more tables together. For example:
CREATE VIEW order_details AS SELECT orders.order_id, customers.customer_name, orders.order_date FROM orders INNER JOIN customers ON orders.customer_id = customers.customer_id;
This view would create a virtual table called "order_details" that contains the order ID, customer name, and order date columns from the "orders" and "customers" tables, joined together on the customer ID column.
- Filtered view: This is a view that filters data based on specific criteria. For example:
CREATE VIEW high_value_customers AS SELECT customer_id, customer_name FROM orders INNER JOIN customers ON orders.customer_id = customers.customer_id WHERE order_total > 2000;
This view would create a virtual table called "high_value_customers" that contains the customer ID and customer name columns from the "customers" table, but only for customers who have placed orders with a total value greater than $2000.
- Aggregated view: This is a view that aggregates data using functions like SUM, AVG, and COUNT. For example:
CREATE VIEW order_totals AS SELECT customer_id, SUM(order_total) AS total_sales FROM orders GROUP BY customer_id;
This view would create a virtual table called "order_totals" that contains the customer ID and total sales columns, where the total sales are calculated by summing up the order_total column for each customer.
5.Creating a simple view that selects columns from a table:
CREATE VIEW customer_details AS SELECT customer_id, first_name, last_name, email FROM customers;
This view would select the "customer_id", "first_name", "last_name", and "email" columns from the "customers" table, and create a virtual table called "customer_details".
6.Creating a view that joins multiple tables:
CREATE VIEW order_details AS SELECT orders.order_id, orders.order_date, customers.first_name, customers.last_name FROM orders JOIN customers ON orders.customer_id = customers.customer_id;
This view would join the "orders" and "customers" tables on the "customer_id" column, and select the "order_id", "order_date", "first_name", and "last_name" columns from the result set.
7.Creating a view with a WHERE clause:
CREATE VIEW recent_orders AS SELECT order_id, order_date, customer_id FROM orders WHERE order_date > '2023-02-03';
This view would select the "order_id", "order_date", and "customer_id" columns from the "orders" table, but only include orders that were placed after February 3rd, 2023.
8.Creating a view with calculated columns:
CREATE VIEW sales_summary AS SELECT product_id, SUM(quantity) AS total_quantity, SUM(price * quantity) AS total_sales FROM order_items GROUP BY product_id;
Views are a potent feature tool of SQL that can be used to enhance security, create virtual tables, provide a consistent interface, restrict access to data, aggregate data, combine data from multiple tables, and provide a consistent interface to data.
Views can be tailored to meet the requirements of a particular database and application, and they can be used to give your database's data a standardized interface.
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