Database develop. life cycle - Database Views and Their Applications

A database view is a virtual table created from one or more existing tables in a database. Unlike a physical table, a view generally does not store the actual data separately. Instead, it stores a SQL query that defines which data should be displayed. Whenever a user accesses the view, the database retrieves the required information from the underlying tables. Views are useful for simplifying complex queries, controlling access to sensitive information, and presenting data in a format that is easier for users and applications to understand.

Creating a Database View

A view is commonly created using the CREATE VIEW statement. The query inside the statement determines the columns and rows that will be available through the view.

For example, consider an Employees table containing the following information:

CREATE TABLE Employees (
    EmployeeID INT,
    EmployeeName VARCHAR(100),
    Department VARCHAR(50),
    Salary DECIMAL(10,2),
    Email VARCHAR(100)
);

If an organization wants employees to access only their names, departments, and email addresses without exposing salary information, a view can be created:

CREATE VIEW EmployeeContact AS
SELECT EmployeeID, EmployeeName, Department, Email
FROM Employees;

Users can then retrieve information from the view using a normal SELECT statement:

SELECT * FROM EmployeeContact;

The result appears similar to a table, but the data is obtained from the underlying Employees table.

Simplifying Complex Queries

One of the major applications of views is simplifying complicated SQL queries. A database may contain several related tables, requiring joins, filtering, grouping, and calculations to retrieve useful information. Instead of requiring users to write the complete query repeatedly, a view can store that query.

For example:

CREATE VIEW DepartmentEmployeeCount AS
SELECT Department, COUNT(*) AS EmployeeCount
FROM Employees
GROUP BY Department;

Users can simply execute:

SELECT * FROM DepartmentEmployeeCount;

This makes complex database operations easier for application developers, analysts, and other users who may not need to understand the underlying database structure.

Improving Data Security

Views can also provide an additional layer of data access control. A database may contain confidential information that should not be available to every user. Instead of giving users direct access to the entire table, administrators can provide access to a view containing only the permitted columns or rows.

For example:

CREATE VIEW PublicEmployeeDetails AS
SELECT EmployeeID, EmployeeName, Department
FROM Employees;

The salary and email columns are excluded from this view. A user who has permission to access the view can see only the information exposed by it.

Views can also restrict rows. For example:

CREATE VIEW SalesDepartmentEmployees AS
SELECT EmployeeID, EmployeeName, Department
FROM Employees
WHERE Department = 'Sales';

This view displays only employees belonging to the Sales department.

It is important to note that views are not, by themselves, a complete security mechanism. Proper database privileges and access controls should still be configured.

Providing a Consistent Data Interface

Database structures may change over time. Tables can be reorganized, columns can be renamed, or additional tables can be introduced. A view can provide applications with a stable interface even when the underlying database structure changes, provided the changes do not invalidate the view.

For example, an application may always use:

SELECT EmployeeName, Department
FROM EmployeeContact;

The application does not necessarily need to know how the underlying employee information is organized.

This can reduce the dependency between applications and the physical organization of database data.

Combining Multiple Tables

A view can retrieve information from multiple tables using joins. This is particularly useful when users frequently need related information from different parts of a database.

Suppose there are Employees and Departments tables:

CREATE VIEW EmployeeDepartmentDetails AS
SELECT
    Employees.EmployeeName,
    Departments.DepartmentName
FROM Employees
JOIN Departments
ON Employees.DepartmentID = Departments.DepartmentID;

Users can access the combined information through:

SELECT * FROM EmployeeDepartmentDetails;

This avoids requiring every user or application to repeatedly write the join operation.

Updating Data Through Views

Some database views can be used not only for reading data but also for modifying the underlying tables. For example, a simple view based on a single table may allow INSERT, UPDATE, or DELETE operations, depending on the database system and the definition of the view.

For example:

CREATE VIEW EmployeeBasicDetails AS
SELECT EmployeeID, EmployeeName, Department
FROM Employees;

An update such as:

UPDATE EmployeeBasicDetails
SET Department = 'Finance'
WHERE EmployeeID = 101;

may update the corresponding record in the underlying Employees table when the view is updatable.

However, not every view is updatable. Views involving operations such as complex joins, aggregation, grouping, or certain calculated expressions may have restrictions on modification.

Types of Views

Views can be broadly understood according to how they are constructed and used.

A simple view is generally based on a single table and contains straightforward columns and filtering conditions.

A complex view may involve multiple tables, joins, aggregate functions, grouping, calculations, or other advanced SQL operations.

A materialized view is different from a conventional virtual view. It stores the result of a query physically so that frequently requested complex data can be retrieved more quickly. Because the stored result can become outdated when the underlying data changes, materialized views usually require a refresh mechanism.

For example, a reporting system could maintain a materialized view containing monthly sales totals rather than calculating those totals from millions of transaction records every time a report is requested.

Advantages of Database Views

Database views provide several important benefits:

  1. Simplified queries: Complex SQL operations can be hidden behind a simple view name.

  2. Improved security: Sensitive columns and rows can be excluded from the information exposed to users.

  3. Data abstraction: Users can work with a logical representation without needing to understand the complete underlying database structure.

  4. Reusability: Frequently used queries can be defined once and reused by multiple applications or users.

  5. Consistency: Organizations can provide a standardized way of accessing commonly used information.

  6. Reduced application complexity: Applications can retrieve prepared data without repeatedly implementing complicated SQL joins and filtering logic.

Limitations of Database Views

Views also have certain limitations. A view based on complex queries may be expensive to execute, particularly when it is accessed frequently. Some views cannot be updated directly because of their structure. Changes to underlying tables can also cause a view to become invalid or require modification.

Another important consideration is that a conventional view does not automatically improve query performance. Since the database generally executes the underlying query when the view is accessed, performance still depends on the query, indexes, database engine, and amount of data involved.

Conclusion

Database views provide a logical layer between users or applications and the underlying database tables. They are particularly useful for simplifying complex queries, restricting access to selected data, combining information from multiple tables, and presenting data in a consistent format. Views can make database systems easier to manage and use while reducing unnecessary exposure of sensitive information. For reporting systems and frequently accessed complex queries, materialized views can also be used when supported by the database system.