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CIT 241 Chapter 2 Notes

by: Adriana

CIT 241 Chapter 2 Notes CIT 241

Marketplace > Pace University > Information technology > CIT 241 > CIT 241 Chapter 2 Notes
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About this Document

This covers what we learned 2/2 & 2/4 in class
Database Management
Class Notes
CIT, 241, Coakley, Database, Management




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This 6 page Class Notes was uploaded by Adriana on Saturday January 30, 2016. The Class Notes belongs to CIT 241 at Pace University taught by Coakley in Spring 2016. Since its upload, it has received 22 views. For similar materials see Database Management in Information technology at Pace University.

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Date Created: 01/30/16
CIT 241 CHAPTER 2 Entity  something of importance to a user that needs to be represented in a database  An entity represents one theme or topic  In an entity-relationship model,(see ch.4) entities are restricted to things that can be represented by a single table o E.g. A product entity would just have product data Relation  2D table that has specific characteristics  Table dimensions consist of rows and columns Keys A key is one (or more) column(s) of a relation that is/are used to identify a row Unique Key Data value is unique for each row The key will uniquely identify a row Nonunique Key Data value may be shared among multiple rows Composite Key  A key that contains 2 or more attributes  For a key to be unique it must often be a composite key Candidate Key -A unique key that is a candidate to become the primary key Primary Key  A candidate key chosen to be the main key for the relation  If you know the value of the primary key, you will be able to uniquely identify a single row Defining a Primary Key in Microsoft Access 2013 Surrogate Key  A surrogate key is a unique, numeric value that is added to a relation to serve as the primary key.  Surrogate key values have no meaning to users and are usually hidden on forms, queries, and reports.  A surrogate key is often used in place of a composite primary key. Example:  If the Family Member primary key is FamilyID, FirstName, Suffix, it would be easier to append and use a surrogate key of FamMemberID.  FamilyID, FirstName and Suffix remain in the relation. Relationship between Tables  A table may be related to other tables.  For example o An Employee works in a Department o A Manager controls a Project Foreign Key  To preserve relationships, you may need to create a foreign key.  A foreign key is a primary key from one table placed into another table.  The key is called a foreign key in the table that received the key. Example: Referential Integrity Constraint  Referential integrity constraint states that every value of a foreign key must match a value of an existing primary key. Example (see above):  If EmpID = 4 in EMPLOYEE has a DeptID = 7 (a foreign key), a Department with DeptID = 7 must exist in DEPARTMENT.  The primary key value must exist before the foreign key value is entered. Another perspective… The value of the Foreign Key EmployeeID in EQUIPMENT must exist in The values of the Primary Key EmployeeID in EMPLOYEE Enforcing Referential Integrity in Microsoft Access 2013 The Null Value  A Null value means that no data was entered.  This is different from a zero, space character, or tab character. Problem of Null Values  A Null is often ambiguous. It could mean… o The column value is not appropriate for the specific row. o The column value is not decided. o The column value is unknown.  Each may have entirely different implications. Functional Dependency  Functional Dependency—A relationship between attributes in which one attribute (or group of attributes) determines the value of another attribute in the same table  Illustration… o The price of one cookie can determine the price of a box of 12 cookies. (CookiePrice, Qty) --> BoxPrice Candidate/Primary Keys and Functional Dependency  By definition… A candidate key of a relation will functionally determine all other attributes in the row.  Likewise, by definition… A primary key of a relation will functionally determine all other attributes in the row. Example: (EmployeeID) --> (EmpLastName, EmpPhone) (ProjectID) --> (ProjectName, StartDate) Normalization  Normalization—A process of analyzing a relation to ensure that it is well formed  More specifically, if a relation is normalized (well formed), rows can be inserted, deleted, or modified without creating update anomalies. Principles  Relational design principles for normalized relations: o To be a well-formed relation, every determinant must be a candidate key. o Any relation that is not well formed should be broken into two or more well-formed relations. Modification Problems  Tables that are not normalized will experience issues known as modification problems.  Insertion problems o Difficulties inserting data into a relation  Modification problems o Difficulties modifying data into a relation  Deletion problems o Difficulties deleting data from a relation Solving Modification Problems  Most modification problems are solved by breaking an existing table into two or more tables through a process known as normalization. Definitions Functional Dependency  The relationship (within the relation) that describes how the value of one attribute may be used to find the value of another attribute. Determinant  The attribute that can be used to find the value of another attribute in the relation  The right-hand side of a functional dependency Candidate key  The value of a candidate key can be used to find the value of every other attribute in the table.  A simple candidate key consists of only one attribute.  A composite candidate key consists of more than one attribute. Normalization to Boyce-Codd Normal Form (BCNF)  For our purposes, a relation is considered normalized when every determinant is a candidate key. Alternate Terminology Table Row Column File Record Field Relation Tuple Attribute


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