DATA INTIGRITY AND CONTRAINTS

Security constraints guard against accidental or malicious tampering with data, whereas integrity constraints ensure that any properly authorized access, alteration, deletion, or insertion of the data in the database does not change the consistency and validity of the data. This requires that there is a need for guarding against invalid database operations. An operation here is used to indicate any action performed on behalf of a user or application program that modifies the state of the database. Such operations are the result of actions such as update, insert, or delete. In short, invalid changes have to be guarded against by the security subsystem.

Database integrity involves the correctness of data; this correctness has to be preserved in the presence of concurrent operations, errors in the user’s operations and application programs, and failures in hardware and software. Two facets of maintaining the integrity of data in the presence of concurrent operations and failures of various types. For example, the concurrency control mechanism ensures that two concurrent transactions are serialized. However, the integrity constraints must be applied to both these concurrent operations and these constraints ensures that each of these transactions, when run to completion, concurrently or in isolation, will not cause the database to become invalid. The recovery subsystem ensures that failures of various types, which may cause the loss of some of the actions of one or more transactions, will not cause the database to become inconsistent. We consider some type of constraints that the database has to enforce to maintain the consistency and validity of data. One aspect that has to be dealt with by the integrity subsystem is to ensure that only valid values can be assigned to each data-item. This is referred to as domain integrity. Another set of integrity constraints are the so-called structural and semantic constraints. Some of these types of constraints are addressed by the data models used and others are addressed in the design of the database by combining appropriate functional dependencies indifferent records. Some if not most of the functional dependencies can be expressed if the DBMS allows each record type or relation to have an associated primary key.

In traditional system, application programs were responsible for the validation of data and maintaining the consistency of the data used by the programs. However, depending on data application programs to perform these checks has the following drawback.

· Each application program must have correct validation and consistency check routines; a failure in one program could lead to database inconsistencies.

· Each application program must be aware of the semantic of the complete database to enforce the correct consistency checks; this is not always the case and unnecessarily burdens the application program writers.

· There will be considerable duplication of efforts.

· Integrity constraints are hard to understand when they are buried in the code of application programs.

· No consistency or validity checks are possible for direct database manipulation using a query language.

Centralizing the integrity checks directly under the DBMS reduces duplication and ensures the consistency and validity of the database. The centralized integrity constraints can be maintained in a system catalog and

can be accessible to the database users via the query language. This does not rule out an application program performing some specific checking, including input validation.

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