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Deletion anomalies
If we delete a record from the StaffBranch table that represents the last member of staff located at a branch, the details about that branch are also lost from the database. For example, if we delete the record for staff Art Peters (S0415) from the StaffBranch table, the details relating to branch B003 are lost from the database. The design of the tables in Figure 8.1 avoids this problem because branch records are stored separately from staff records and only the column branchNo relates the two tables. If we delete the record for staff Art Peters (S0415) from the Staff table, the details on branch B003 in the Branch table remain unaffected.
Modification anomalies
If we want to change the value of one of the columns of a particular branch in the StaffBranch table, for example the telephone number for branch B001, we must update the records of all staff located at that branch. If this modification is not carried out on all the appropriate records of the StaffBranch table, the database will become inconsistent. In this example, branch B001 would have different telephone numbers in different staff records.
The above examples illustrate that the Staff and Branch tables of Figure 8.1 have more desirable properties than the StaffBranch table of Figure 8.2. In the following sections, we examine how normal forms can be used to formalize the identification of tables that have desirable properties from those that may potentially suffer from update anomalies.
kevowmuchiri answered the question on July 9, 2018 at 18:59
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