Technical Perspective: Attacking the Problem of Consistent Query Answering

Inconsistent data refers to data that do not adhere to one or more constraints. The term constraints refers to conditions that need to be imposed on the data. Constraints often arise from organizational requirements or business logic, such as the requirement that every employee in the database must be uniquely identified by the employee id, or every employee must work on some project, or the expenses cannot exceed the credit limit, or even a desired designated format for storing phone numbers. The need to manage inconsistent data arises in many settings. Quite typically, when one integrates data from different sources, the integrated data can be inconsistent data even when the data sources may be individually consistent. Another scenario where inconsistency in data can arise is when data and/or schema evolves, for example, through the addition or removal of data, changes in schema, or knowledge of new constraints.