Data redundancy exists when the same data are stored unnecessarily at different places.
Uncontrolled data redundancy sets the stage for:
- Poor data security. Having multiple copies of data increases the chances for a copy of the data to be susceptible to unauthorized access. Chapter 15, Database Administration and Security, explores the issues and techniques associated with securing data.
- Data inconsistency. Data inconsistency exists when different and conflicting versions of the same data appear in different places. For example, suppose you change an agent’s phone number or address in the AGENT file. If you forget to make corresponding changes in the CUSTOMER file, the files contain different data for the same agent. Reports will yield inconsistent results that depend on which version of the data is used.
Data that display data inconsistency are also referred to as data that lack data integrity. Data integrity is defined as the condition in which all of the data in the database are consistent with the real-world events and conditions. In other words, data integrity means that
- Data are accurate—there are no data inconsistencies
- Data are verifiable—the data will always yield consistent results.