Views In SQL| Types Of Views In SQL Server

In this article, We’ll go over the different types of views in SQL Server through examples. In addition, we will learn how to create, alter, update, and delete views in SQL Server.

What is a view in SQL?

Views in SQL Server are nothing more than virtual tables based on the result sets of SQL statements from one or more tables.

Views can be used in SQL to provide a security mechanism that allows users to access data through the view without having direct access to the underlying base tables or the actual tables referenced in the view.

• • • In SQL, View is a virtual table of the base table, it has no physical existence.
    • The view only stores select query statements and does not store actual data in the underline base table.
    • You can create views in SQL by selecting columns from one or more tables in the database.
    • Views in SQL can be created with the help of SQL “select statements”.
    • A view is a logical entity, while a table in SQL is a physical entity.
    • Views can be used to provide a backward compatible interface to simulate tables whose schema has changed.

In SQL, views can be used to construct data from one or more tables. It helps to simplify complex SQL queries, prevent users from accessing the actual data in the base table, and summarize the data in multiple tables for reporting.

Views in SQL Server can be used to provide row and column level security. We can allow users to access only the selected columns of the table instead of accessing the entire record in the table.

Syntax to create view in SQL server

The following is the syntax to create a view in the SQL server.

What are the types of views in SQL?

In SQL Server, there are two main types of views available, namely, system-defined views and user-defined views.

• • System Defined Views
    1. 1. Information Schema View
       2. Catalog View
       3. Dynamic Management View
  • User Defined Views
    1. 1. Simple View
       2. Complex View

Let’s first look at the user-defined view.

User Defined Views in SQL

These types of views in SQL are usually user-defined. There are two types of user-defined views, namely simple views and complex views.

Simple View In SQL

In SQL, views created on a single table are called simple views.
Simple views support DML operations. If the base table contains a primary key and no empty columns, we can insert, update or delete records.

How to create a simple view in SQL?

Let’s first create a table with a small number of records, and then let’s create a simple view on that table.

The following is an example of creating a simple view on the SQL server.

-- Creating a new table "tblEmployee".
CREATE TABLE tblEmployee
(
  [Id] INT PRIMARY KEY,
  [Name] NVARCHAR(30),
  [Gender] VARCHAR(10),
  [City] VARCHAR(50),
  [DeptId] INT
);

--Inserting data into the "tblEmployee" table.

INSERT INTO tblEmployee VALUES (101,'Shekh Ali','Male','Delhi', 1),
                               (102,'Mark Adam','Male','New York', 1),
							   (103,'Julie Folden','Female','Chicago', 2);

-- SQL Create a Simple View on base table "tblEmployee"

CREATE VIEW [SimpleViewOnTblEmployee]
AS
SELECT Id, Name, Gender, City, DeptId
FROM tblEmployee;

Let’s query view to show the records like a table.

-- Show records from a simple view like a table.

SELECT * FROM [SimpleViewOnTblEmployee];

Insert operation on Simple view in SQL

In a simple view, we can insert, update or delete records. The following is a query to insert a new record in a simple view.

-- Insert record into Simple view

INSERT INTO [SimpleViewOnTblEmployee] VALUES (104,'Salim Khan','Male','Delhi', 1);

-- Let's see the affected view

SELECT * FROM [SimpleViewOnTblEmployee];

Update operation on simple view in SQL

When we execute the following update query, It will successfully update the record from the view. Let’s update the record whose id is 104.

-- Update record into Simple view

UPDATE [SimpleViewOnTblEmployee] SET Name='Daniel' WHERE Id = 104;

To check the updated record, a select query against the simple view “SimpleViewOnTblEmployee” is shown below.

-- Select record from the sample view
SELECT * FROM [SimpleViewOnTblEmployee] ;

Delete operation on simple view in SQL

Let’s execute the following SQL command to delete the record whose Id is 102 from the simple view.

-- Delete data from view "SimpleViewOnTblEmployee"
Delete from [SimpleViewOnTblEmployee] where Id = 102 ;

-- Let's see the affected view after delete operation
Select * from [SimpleViewOnTblEmployee] ;

In the above image, we can see that the record whose Id was 102 is deleted successfully.

Complex View In SQL

A View created on more than one base table is known as a  Complex View  . Complex view doesn’t allow to perform DML operation. It is also known as a non-updatable or static view.

The complex view can contain a group by clause, order by clause, Aggregate Functions, or join conditions.

-- Create a new [DEPARTMENT] table.

CREATE TABLE DEPARTMENT
(
	[Id] [int] PRIMARY KEY NOT NULL,
	[DeptName] [nvarchar](50) NULL,
);

-- INSERT RECORD INTO THE DEPARTMENT TABLE.

INSERT INTO DEPARTMENT VALUES(1,'IT'),(2,'HR'),(3,'FINANCE');

-- Show record from the department table.
SELECT * FROM DEPARTMENT;

-- Now create view on two tables [tblEmployee] and [Department]
CREATE VIEW vw_tblEmployee_Department
AS
SELECT e.Id, e.Name,e.Gender,e.City,d.DeptName
FROM tblEmployee e INNER JOIN Department d
ON e.DeptId = d.Id;

Now run the below query to show the records from the [vw_tblEmployee_Department] complex view.

-- Run the below query to show the records from the complex view [vw_tblEmployee_Department] .
SELECT * FROM vw_tblEmployee_Department;

Update Complex View in SQL

Note: We are allowed to update the existing records in a complex view, but can’t insert the new records.

If a complex view is based on multiple tables, might be it will not allow updating the base table correctly. To update the complex view, instead of trigger can be used.

Following is the SQL query to update the complex view.

-- Query to update [City] in the in the complex view [vw_tblEmployee_Department].
UPDATE vw_tblEmployee_Department set City ='Mumbai' Where Id = 101;

Once we run the above query, we can see that the City is updated from “Delhi” to “Mumbai” for Id 101.

System Defined Views In SQL

In SQL Server, the  System defined views  are predefined Views that already exist in the master database.

The master database generally serves as the template database for all other user-defined databases on the SQL server. It contains almost 230 predefined views.
The following are three system-defined views, namely the information schema view, the catalog view, and the dynamic management view.

Information Schema View In SQL ServerS nII LSn n

In SQL Server, there are approx 21 different Information schema views are available. These types of views start with  INFORMATION_SCHEMA  as prefix and used to provide information about the database objects such as tables, columns, domains, check constraints etc.
Following is the SQL statement to view detailed information of the columns of table tblEmployee.

-- To view detailed information of the columns of table tblEmployee

SELECT * FROM INFORMATION_SCHEMA.COLUMNS
where TABLE_NAME='tblEmployee';

-- OR

SELECT TABLE_NAME,COLUMN_NAME,DATA_TYPE,CHARACTER_MAXIMUM_LENGTH FROM INFORMATION_SCHEMA.COLUMNS
where TABLE_NAME='tblEmployee';

The following INFORMATION_SCHEMA.CHECK_CONSTRAINTS is used to return the information about the constraints of a table.

-- Return information about the constraints on a table

SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS  
WHERE TABLE_NAME ='tblEmployee'; 

The following INFORMATION_SCHEMA.VIEWS will return all the views present in the current database.

--This INFORMATION_SCHEMA.VIEWS will return all the views present in the current database. 
SELECT * FROM INFORMATION_SCHEMA.VIEWS ;

What are Catalog Views in SQL Server?

As per MSDN: Catalog views are used to return information used by the SQL server database engine. Catalog views are mainly used to provide information about a database metadata and its objects such as tables and views.
The  sys.objects  catalog view is referred as the base view, and the  sys.tables  view is called the derived view. The sys.tables catalog view in SQL server returns the columns that are specific to tables and also all the columns that the sys.objects catalog view returns.

The sys.objects catalog view returns rows of objects other than tables, such as stored procedures and views. Once a new table is created, the metadata for the table is returned in both sys.objects and sys.tables views.

Use of catalog views in SQL Server:

• • Catalog views provide information about the metadata of SQL databases.
  • Catalog views returns rows of objects other than tables, such as stored procedures and views.
  • Catalog views provide descriptive column names, which might be very helpful for users to understand the data.

-- Catalog view example for tables:
SELECT * FROM sys.tables;

-- Catalog view example for database
SELECT name, database_id, compatibility_level,*
FROM sys.databases
WHERE name LIKE 'master%';

What Is Dynamic Management Views In SQL Server?

The  Dynamic Management Views  were introduced in SQL Server 2005. These Views provide the administrator information about the server state to diagnose problems, monitor the health and the current-state of the SQL server machine.
These views help the administrator to analyze database-related problems and tune the SQL server for optimal performance.

These are two types of Dynamic Management View.

Server Scoped Dynamic Management View: These types of views are stored only in the Master database.

Database Scoped Dynamic Management View: These types of views are stored in each database.

--This returns current session information 
SELECT login_name ,COUNT(session_id) AS session_count  
FROM sys.dm_exec_sessions  
GROUP BY login_name;

--To see all SQL Server connections
SELECT connection_id,session_id,client_net_address,auth_scheme
FROM sys.dm_exec_connections ;

FAQs

Conclusion

In this article, we discussed different types of views in SQL, such as System Defined Views and User Defined Views.

I hope you like this post. If you have any questions, please post your question or suggestion in the comment section.

References w3schools: SQL Views

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