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Chapter 10: Problem 3

Why should nulls in a relation be avoided as far as possible? Discuss the problem of spurious tuples and how we may prevent it.

Short Answer

Expert verified
Nulls in a relation should be avoided as they can lead to uncertainty in comparison, data integrity issues and can even influence system performance. Spurious tuples, occurring due to improper join operations causing additional rows that did not exist in original relations, can produce inaccurate results. They can be prevented by proper join conditions, including all necessary attributes, and database schema normalization.

Step by step solution

01

Defining Nulls in a Relation

Null in a relational database is a value that is undefined or unknown. It is not equivalent to zero or blank, but rather signifies the absence of a value. Here we will explain why these null values should be carefully managed or avoided altogether.
02

Explaining the Problems caused by Nulls

Nulls can lead to several issues in databases. They make comparison uncertain and cause problems with data integrity and database operations like Select, Insert, Update and Delete. For some database systems, Nulls can have an impact on how the system processes information and can affect the performance of the system. Additionally, null values can lead to inaccurate results when performing calculations or data analysis.
03

Understanding Spurious Tuples

Spurious tuples are 'extra' tuples (rows of data in a table) that appear when two relations are joined improperly. They are called so because they do not exist in the original relations but yet appear in the result of an incorrect join operation. This can cause confusion as well as inaccurate analysis or reports.
04

Preventing Spurious Tuples

To prevent spurious tuples, we must ensure that we properly join tables. This often means including all the necessary attributes in the join condition and ensuring that these attributes match appropriately. Normalization of database schemas can also prevent spurious tuples by reducing redundancy and properly structuring the database.

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Most popular questions from this chapter

Define first, second, and third normal forms when only primary keys are considered. How do the general definitions of \(2 \mathrm{NF}\) and \(3 \mathrm{NF}\), which consider all keys of a relation, differ from those that consider only primary keys?

Consider the following two sets of functional dependencies: \(F=\\{A \rightarrow C, A C \rightarrow\) \(D, E \rightarrow A D, E \rightarrow H\\}\) and \(G=\\{A \rightarrow C D, E \rightarrow A H\\} .\) Check whether they are equivalent.

Suppose that we have the following requirements for a university database that is used to keep track of students' transcripts: a. The university keeps track of each student's name (SNAME), student number (SNUM), social security number (SSN), current address (SCADDR) and phone \((\mathrm{SCPHONE}),\) permanent address \((\mathrm{SPADDR})\) and phone \((\mathrm{SPPHONE}),\) birth date \((\mathrm{BDATE})\) \(\operatorname{sex}(\operatorname{sex}), \text { class (cLass) (freshman, sophomore, } \ldots, \text { graduate }),\) major depart ment (MAJORCODE), minor department (MINORCODE) (if any), and degree program \(\left(p_{R O C}\right)(B, A,, B, S, \ldots, P H, D,) .\) Both sss \(N\) and student number have unique val. ues for each student. b. Each department is described by a name (DNAME), department code (DCOOE), office number (DOFFICE), office phone (DPHONE), and college (DCOLLECE). Both name and code have unique values for each department. c. Each course has a course name (cNAME), description (cDESC), course number (CNUM), number of semester hours (cREDIT), level (LEVEL), and offering depart. ment (coept). The course number is unique for each course. d. Each section has an instructor (INAME), semester (SEMESTER), year (YEAR), course (seccourse), and section number (secwum). The section number distinguishes different sections of the same course that are taught during the same semester/ year; its values are \(1,2,3, \ldots,\) up to the total number of sections taught during each semester. e. \(A\) grade record refers to a student \((\operatorname{ss} N),\) a particular section, and a grade \((\mathrm{CRADE})\) Design a relational database schema for this database application. First show all the functional dependencies that should hold among the attributes. Then design relation schemas for the database that are each in \(3 \mathrm{NF}\) or BCNF. Specify the key attributes of each relation. Note any unspecified requirements, and make appropriate assumptions to render the specification complete.

What undesirable dependencies are avoided when a relation is in \(3 \mathrm{NF}\) ?

What role do Armstrong's inference rules- -the three inference rules IR 1 through IR3-play in the development of the theory of relational design?
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