Purpose of Database Systems

Database systems arose in response to early methods of computerized management of commercial data. An example of such methods,typical of 1960s,consider part of a bank enterprise that,among other data,keeps information about all customer is to store it in operating system files.To allow users to manipulate the information,the system has a number of applicationprograms that manipulate the files including programs to:

• Debit or credit an account
• Add a new account
• Find the balance of an account
• Generate monthly statements
  

System programmers wrote these application programs to meet the needs of the bank.
New application programs are added to the system as the need arises.For example,suppose that a saving bank decides to offer checking accounts.As a result,the bank creates new permanent files that contain information about all the checking accounts maintained in the bank,and it may have to write new application program to deal with situation that do not arisein saving accounts such as overdrafts.Thus,as time goes by,the system acquires more files and more application programs.
This typical file-processing system is supported by a conventional operating system.The system stores permanent recordsin various files and it need various different application programs to extract records from,and add records to,the appropriate file.Before database management systems(DBMSs) came along,organisation usually store information in such system.
Keeping organizational information in a file-processing system has a number of major disadvantages:

• Data redundancy and inconsistency: Same information may be duplicated in several places. All copies may not be updated properly.

• Difficulty in accessing data: May have to write a new application program to satisfy an unusual request. E.g. find all customers with the same postal code. Could generate this data manually, but a long job...

• Data isolation: Data in different files, Data in different formats Difficult to write new application programs.

• Security problems
• Every user of the system should be able to access only the data they are permitted to see.
• E.g. payroll people only handle employee records, and cannot see customer accounts; tellers only access account data and cannot see payroll data.
• Difficult to enforce this with application programs.

• Integrity problems
• Data may be required to satisfy constraints.
• E.g. no account balance below $25.00.
• Again, difficult to enforce or to change constraints with the file-processing approach.