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Database Systems 6th Edition by Ramez Elmasri, Shamkant B. Navathe
Edition 6ISBN: 0136086209Database Systems 6th Edition by Ramez Elmasri, Shamkant B. Navathe
Edition 6ISBN: 0136086209 Exercise 2
What is the difference between controlled and uncontrolled redundancy?
Explanation
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Redundancy is when the same fact is stored multiple times in several places in a database.
For example, in Figure 1.5(a) the fact that the name of the student with StudentNumber=8 is Brown is stored multiple times. Redundancy is controlled when the DBMS ensures that multiple copies of the same data are consistent; for example, if a new record with StudentNumber=8 is stored in the database of Figure 1.5(a), the DBMS will ensure that StudentName=Smith in that record. If the DBMS has no control over this, we have uncontrolled redundancy.
For example, in Figure 1.5(a) the fact that the name of the student with StudentNumber=8 is Brown is stored multiple times. Redundancy is controlled when the DBMS ensures that multiple copies of the same data are consistent; for example, if a new record with StudentNumber=8 is stored in the database of Figure 1.5(a), the DBMS will ensure that StudentName=Smith in that record. If the DBMS has no control over this, we have uncontrolled redundancy.
Database Systems 6th Edition by Ramez Elmasri, Shamkant B. Navathe
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