Review the advantages of database management systems in the Connolly textbook. Which two advantages do you think are the most important? Why? Give an example of a database or application you use that demonstrates those advantages.

Sharing of data

Typically, files are owned by the people or departments that use them. On the other hand, the database belongs to the entire organization and can be shared by all authorized users. In this way, more users share more of the data. Furthermore, new applications can build on the existing data in the database and add only data that is not currently stored, rather than having to define all data requirements again. The new applications can also rely on the functions provided by the DBMS, such as data definition and manipulation, and concurrency and recovery control, rather than having to provide these functions themselves.

Improved data integrity

Database integrity refers to the validity and consistency of stored data. Integrity is usually expressed in terms ofconstraints, which are consistency rules that the database is not permitted to violate. Constraints may apply to data items within a single record or to relationships between records. For example, an integrity constraint could state that a member of staff's salary cannot be greater than $40,000 or that the branch number contained in a staff record, representing the branch where the member of staff works, must correspond to an existing branch office. Again, integration allows the DBA to define integrity constraints, and the DBMS to enforce them.

Improved security

Database security is the protection of the database from unauthorized users. Without suitable security measures, integration makes the data more vulnerable than file-based systems. However, integration allows the DBA to define database security, and the DBMS to enforce it. This security may take the form of user names and passwords to identify people authorized to use the database. The access that an authorized user is allowed on the data may be restricted by the operation type (retrieval, insert, update, delete). For example, the DBA has access to all the data in the database; a branch manager may have access to all data that relates to his or her branch office; and a sales assistant may have access to all data relating to properties but no access to sensitive data such as staff salary details.

enforcement of standards

Again, integration allows the DBA to define and the DBMS to enforce the necessary standards. These may include departmental, organizational, national, or international standards for such things as data formats to facilitate exchange of data between systems, naming conventions, documentation standards, update procedures, and access rules.

economy of scale

Combining all the organization's operational data into one database and creating a set of applications that work on this one source of data can result in cost savings. In this case, the budget that would normally be allocated to each department for the development and maintenance of its file-based system can be combined, possibly resulting in a lower total cost, leading to an economy of scale. The combined budget can be used to buy a system configuration that is more suited to the organization's needs. This may consist of one large, powerful computer or a network of smaller computers.

Balance of conflicting requirements

Each user or department has needs that may be in conflict with the needs of other users. Because the database is under the control of the DBA, the DBA can make decisions about the design and operational use of the database that provide the best use of resources for the organization as a whole. These decisions will provide optimal performance for important applications, possibly at the expense of less-critical ones.

Improved data accessibility and responsiveness

Again, as a result of integration, data that crosses departmental boundaries is directly accessible to the end-users. This provides a system with potentially much more functionality that can, for example, be used to provide better services to the end-user or the organization's clients. Many DBMSs provide query languages or report writers that allow users to askad hocquestions and to obtain the required information almost immediately at their terminal, without requiring a programmer to write some software to extract this information from the database.

Increased productivity

As mentioned previously, the DBMS provides many of the standard functions that the programmer would normally have to write in a file-based application. At a basic level, the DBMS provides all the low-level file-handling routines that are typical in application programs. The provision of these functions allows the programmer to concentrate on the specific functionality required by the users without having to worry about low-level implementation details. Many DBMSs also provide a fourth-generation environment, consisting of tools to simplify the development of database applications. This results in increased programmer productivity and reduced development time (with associated cost savings).

Improved maintenance through data independence

In file-based systems, the descriptions of the data and the logic for accessing the data are built into each application program, making the programs dependent on the data. A change to the structure of the datasuch as making an address 41 characters instead of 40 characters, or a change to the way the data is stored on diskcan require substantial alterations to the programs that are affected by the change. In contrast, a DBMS separates the data descriptions from the applications, thereby making applications immune to changes in the data descriptions.

Increased concurrency

In some file-based systems, if two or more users are allowed to access the same file simultaneously, it is possible that the accesses will interfere with each other, resulting in loss of information or even loss of integrity. Many DBMSs manage concurrent database access and ensure that such problems cannot occur. We discuss concurrency control inChapter 22.

Improved backup and recovery services

Many file-based systems place the responsibility on the user to provide measures to protect the data from failures to the computer system or application program. This may involve performing a nightly backup of the data. In the event of a failure during the next day, the backup is restored and the work that has taken place since this backup is lost and has to be re-entered. In contrast, modern DBMSs provide facilities to minimize the amount of processing that is lost following a failure. We discuss database recovery in