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Views in SQL Server


This article provides an overview of the basic views in SQL Server. The way to view definition, create or replace view view or replace the views in SQL is by using SQL Server Management Studio / SQL-SQL.

What is A View In SQL Server

In SQL Server, a view is a virtual table that is based on the result of a SELECT statement. The view itself does not store data but instead acts as a filter on top of one or more real tables together, allowing you to see only the data from the real table or virtual table together that meets certain criteria.

Once a view is created, you can treat it like a regular table by querying it with SELECT, INSERT, UPDATE, and DELETE statements, although these operations may have some limitations based on the view's definition. Views can be used to simplify complex queries, abstract away unnecessary details, and provide a layer of security by restricting access to certain rows and columns, or restrict access to other rows and columns.

Views can also be used to aggregate data from multiple columns, create or replace view statement between them, join multiple tables into replace view, create or replace view to provide a customized representation of data. In addition, views can be nested within other views, allowing for even greater flexibility in data modeling and query construction.



What are the disadvantages of views in SQL Server

While views can be useful in many situations, there are also some potential disadvantages to using them in SQL Server. These include:

Performance overhead:

Depending on the complexity of the view, querying it may result in additional processing overhead and slower performance than querying a table directly.

Limited write capabilities:

While views can be used to update, insert, and delete data, there are some limitations based on the view's definition. For the above example below, a view may not allow updates to certain columns or rows, or may not allow certain types of joins or aggregations.

Maintenance overhead:

Creating and maintaining views can add to the complexity of creating views in a database schema and increase the amount of work required to manage it.

Difficulty in optimization:

Views may be difficult to optimize for performance, as the query optimizer must consider the underlying tables and the view's view definition, when generating a query plan.

Security concerns:

Views may provide a layer of security by restricting access to certain data, but they may also be used to bypass other security measures if not configured correctly.

It's important to weigh the benefits and drawbacks of using views in the same way in each specific situation and to consider alternative approaches if necessary.

How to create a view SQL Syntax

The basic syntax for creating a view in SQL Server is as follows:

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

In this syntax:

view_name is the name you want to give to the view.

column1, column2, column list, etc. are the names of the columns you want to include in the view.

table_name is the name of the table or tables from create view statement which you want to create view retrieve data for the above sql create view below.

condition is an optional WHERE clause that specifies any conditions that must be met for a row to be created create a view to be included in the view.

Once you have created the view, you can use it in queries just like a regular table. Note that the SELECT statement used to define the view can be any valid SELECT statement, including joins, aggregations, and subqueries.

How to create a view in SQL with a Single Table

Here is an example T-SQL script that creates a table, populates it with 10 rows of sample data, and then creates a simple view, based on that table:

-- Create the table
CREATE TABLE MyTable (
    Id INT IDENTITY(1,1) PRIMARY KEY,
    FirstName VARCHAR(50) NOT NULL,
    LastName VARCHAR(50) NOT NULL,
    Age INT NOT NULL,
    Email VARCHAR(100) NOT NULL
);

-- Insert sample data
INSERT INTO MyTable (FirstName, LastName, Age, Email)
VALUES
('John', 'Doe', 25, 'john.doe@example.com'),
('Jane', 'Doe', 32, 'jane.doe@example.com'),
('Bob', 'Smith', 45, 'bob.smith@example.com'),
('Alice', 'Johnson', 27, 'alice.johnson@example.com'),
('Tom', 'Brown', 40, 'tom.brown@example.com'),
('Sara', 'Williams', 35, 'sara.williams@example.com'),
('David', 'Lee', 28, 'david.lee@example.com'),
('Mary', 'Clark', 30, 'mary.clark@example.com'),
('Peter', 'Adams', 50, 'peter.adams@example.com'),
('Sarah', 'Davis', 42, 'sarah.davis@example.com');

-- Create the view
CREATE VIEW MyView AS
SELECT Id, FirstName, LastName, Age
FROM MyTable;

In this example, the table is called MyTable, with columns for Id, FirstName, LastName, Age, and Email. The table is then populated with 10 rows of sample data using the INSERT INTO statement.


Finally, a simple empty view is created that is created called MyView that selects the Id, FirstName, LastName, and Age columns from MyTable.


ALTER a SQL VIEW

Here's an example of how you could alter the view example. MyView view example made from the previous view example is:

-- Alter the view to include the Email column
ALTER VIEW MyView AS
SELECT Id, FirstName, LastName, Age, Email
FROM MyTable;

This ALTER VIEW statement modifies the MyView view so that it now includes the Email column from the MyTable table. Note that you can add or remove columns from a view using the ALTER VIEW statement in a similar way to how you can either create or replace a view or replace a view using the CREATE VIEW statement.




Updatable Views

Updatable views in SQL Server are views of tables that allow modifications to the underlying data. In other words, they allow you to perform insert, update, and delete operations on the data as if it were a regular table.

To make a view updatable, there are several requirements that must be met:

The view must select from only one base table. If the view is based on multiple base tables, then it must not have any ambiguous joins or other constructs that could lead to uncertainty about which table to select statement create or replace view to update.

The view must select all of the columns from the base table that are involved in the modification. If the view omits any columns, then the modification operation will fail.

The view must not contain any aggregate functions or subqueries in the SELECT clause.

The columns being modified must not be derived columns column names or expressions.

The whole view statement must have a unique index or constraint on it that includes all of the other view statement columns being modified.

here's an example of creating a new created table and an updatable view based on a created view that same created table::

-- Create the base table
CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    FirstName VARCHAR(50) NOT NULL,
    LastName VARCHAR(50) NOT NULL,
    Age INT NOT NULL,
    Department VARCHAR(50) NOT NULL
);

-- Populate the base table with some sample data
INSERT INTO Employees (EmployeeID, FirstName, LastName, Age, Department)
VALUES
(1, 'John', 'Doe', 25, 'Sales'),
(2, 'Jane', 'Doe', 32, 'Marketing'),
(3, 'Bob', 'Smith', 45, 'IT'),
(4, 'Alice', 'Johnson', 27, 'HR'),
(5, 'Tom', 'Brown', 40, 'IT'),
(6, 'Sara', 'Williams', 35, 'Marketing');

-- Create an updatable view based on the base table
CREATE VIEW EmployeeView AS
SELECT EmployeeID, FirstName, LastName, Age, Department
FROM Employees
WHERE Department = 'IT';

In this following example below, we first create a base table called Employees with columns for EmployeeID, FirstName, LastName, Age, and Department. We then populate the table with some sample data using the INSERT INTO statement.

Finally, we create an updatable view in sql, called EmployeeView that selects only the employees in the IT department. This view meets the requirements for updatable views, as it selects from a single base table, selects all of null values in the table columns not involved in the modification, and does not contain any aggregate functions or subqueries in the SELECT clause.

Now, if we were to perform an insert, update, or delete operation on the EmployeeView, the corresponding operation would be performed on the Employees table, allowing us to modify all the data in the same table, through the same view again.

Here are some examples of how you could use the updatable EmployeeView view from the previous example to perform insert, update, and delete operations on the underlying Employees table:

-- Insert a new employee into the IT department using the view
INSERT INTO EmployeeView (EmployeeID, FirstName, LastName, Age, Department)
VALUES (7, 'Alex', 'Lee', 30, 'IT');

-- Update the age of an existing employee in the IT department using the view
UPDATE EmployeeView
SET Age = 31
WHERE EmployeeID = 3;

-- Delete an employee from the IT department using the view
DELETE FROM EmployeeView
WHERE EmployeeID = 5;


In these examples, we use the INSERT INTO, UPDATE, and DELETE statements to modify the data in the Employees table through the EmployeeView view. Note that the modifications are performed on the base table, not the view itself. The view is simply in such a way as to provide a different perspective on the data in the base table.

Conditions for Creating Partitioned Views

A Partitioned View in SQL Server is a database object that logically groups together multiple tables with column names and similar structures and allows users to query the data in these tables as if they were a single table. The tables that are included in the partitioned view can be partitioned horizontally, vertically, or both.

Horizontal partitioning divides the data based on rows, while vertical partitioning divides the data based on columns. Both techniques can be used to optimize query performance and manage large amounts of data.

A partitioned view appears as a single table to users and applications, even though it is composed of multiple underlying one or more tables. When a query is executed against the partitioned view, SQL Server automatically routes the query to the appropriate underlying table or one or more tables, to retrieve the required data.

Partitioned views have several advantages and disadvantages that should be considered before implementing them in a SQL Server database:

Advantages:

Simplified Data Management:

Partitioned views allow you to logically group multiple tables with similar structures and query them as a single object. This can simplify data management by reducing the need virtual table, for complex joins and other query techniques.

Improved Query Performance:

Partitioned views can improve query performance by routing queries to the appropriate underlying table or tables. This can help to reduce the amount of data that needs to be processed and improve query response times.

Flexible Partitioning:

Partitioned views support both horizontal and vertical partitioning, which can help to optimize performance and manage large amounts of data. This flexibility allows you to tailor the partitioning strategy of create view to meet the specific needs of your application.

Transparent to Applications:

Partitioned views are transparent to applications, which means that you can implement partitioning without changing your existing code or queries. This can simplify the process of implementing partitioning in an existing database.


Disadvantages:

Limited Functionality:

Partitioned views have some limitations and are not suitable for all scenarios. For example, they cannot be used with certain types of SQL Server features, such as indexed views and full-text search.

Complex Design:

Partitioned views can be complex to design and implement, especially when dealing with large amounts of data. Careful planning and testing are required to ensure that the partitioning strategy is effective and optimized for your specific needs.

Maintenance Overhead:

Partitioned views require additional maintenance overhead, such as managing the underlying tables and ensuring that the partitioning strategy remains effective over time. This can add complexity and increase the workload of database administrators.

here's an example of a Partitioned View in SQL Server using the two tables named "Orders2019" and customers table name "Orders2020":

CREATE TABLE Orders2019
(
    OrderID int PRIMARY KEY,
    CustomerID int,
    OrderDate datetime,
    TotalAmount decimal(10,2)
);

CREATE TABLE Orders2020
(
    OrderID int PRIMARY KEY,
    CustomerID int,
    OrderDate datetime,
    TotalAmount decimal(10,2)
);

-- Sample data for Orders2019 table
INSERT INTO Orders2019 (OrderID, CustomerID, OrderDate, TotalAmount)
VALUES (1, 1001, '2019-01-01', 100.00),
       (2, 1002, '2019-01-05', 200.00),
       (3, 1001, '2019-01-10', 300.00),
       (4, 1003, '2019-01-15', 400.00);

-- Sample data for Orders2020 table
INSERT INTO Orders2020 (OrderID, CustomerID, OrderDate, TotalAmount)
VALUES (5, 1002, '2020-01-01', 500.00),
       (6, 1001, '2020-01-05', 600.00),
       (7, 1003, '2020-01-10', 700.00),
       (8, 1002, '2020-01-15', 800.00);

-- Create a Partitioned View
CREATE VIEW OrdersView
AS
SELECT * FROM Orders2019
UNION ALL
SELECT * FROM Orders2020;

Now, we can query the "OrdersView" just like a regular table to retrieve data from both "Orders2019" and "Orders2020" tables at the same time, following example:

SELECT * FROM OrdersView;


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