chapter1 part2 - Structural Data Dependence Chapter 1 File...

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Chapter 1 Database Systems part 2 BUS 173 Spring 2010 Dr. Hamouda Structural & Data Dependence File System exhibits: 1. Structural dependence : access to a file is dependent on its own structure All file system programs must be modified to conform to a new file structure Structural independence : changing file structure does not affect the application program ability to access the data 2. Data dependence : data access changes data when storage characteristics change Ex. change data type from integer to real Data independence : data storage characteristics do not affect data access 30 Data dependence makes the system complex and extremely cumbersome To access the data, each program must have instructions on what and how to do: What file to open What record to read What field to read along with its definition 31 Structural & Data Dependence Data Redundancy Data redundancy : same data stored unnecessarily in different places Causes of data redundancy File system structure makes it difficult to combine data from multiple sources Organizational structure promotes storage of same data in different locations Data stored in different locations is unlikely to be updated consistently 32 Data Redundancy Data Redundancy may cause: 1. Data inconsistency : different and conflicting versions of same data occur at different places 2. Data anomalies : abnormalities that occur when changes in redundant data are not made correctly Update anomalies Insertion anomalies Deletion anomalies 33 34 Example of anomalies Data Redundancy
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Database Systems Unlike the FS (with separate and unrelated files), a DB system consists of logically related
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