Relational Databases
A relational database is one in which you can store information in different tables, each containing different information that relates to information in the other tables
Definitions
- a database is a collection of related data
- a database management system (or DBMS) is a system designed for two main purposes
- to add, delete, and update data in the database
- to provide various ways to view (on screen or in print) the data in the database
- a flat file database is one where all the data is contained in a single table, such as in Excel
- a relational database is one in which you can store information in different tables, each containing different information that relates to information in the other tables
- comparing flat file databases to relational databases
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Why use a relational database design?
- to eliminate unnecessary redundancy or repetition in storage and in steps required for database maintenance
- to be able to distribute the database to multiple locations, each with a responsibility to maintain a part of the total database
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So a relational database is characterized by
- multiple tables, perhaps maintained by multiple locations
- with relationships defined between the tables, so that the entire distributed, relational database works together
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Entity Relationship Model of a Database
an entity-relationship model (ERM) is an abstract and conceptual representation of data. Entity-relationship modeling is a database modeling method, used to produce a type of conceptual schema or semantic data model of a system, often a relational database, and its requirements in a top-down fashion. Diagrams created by this process are called entity-relationship diagrams (or ER diagrams)
One has choices in how to model the elements in a database. One of the standard textbooks on the topic (Elmasri, R., & Navathe, S. (2011). Fundamentals of database systems. Boston: Addison-Wesley.) offers a view of the several methods.
The purpose of a database is to store information about certain types of objects.
- In database language, objects are entities;
- in Access, entities are records
An entity class [like books] is an abstract definition of something
- an entity (like the textbook for INLS261) is a concrete example of that description;
- in Access, an entity is a record
An entity class is composed of attributes
- in Access, attributes are fields
- they include information we want in the database
- help to uniquely identify individual entities within a class
- can describe relationships between entities in different classes
An entity set is the list of given entities/records within a given entity class that are currently in the database/table
- in Access, an entity set is a table
Primary and Foreign Keys
- a Primary Key is a set of attributes/fields that uniquely identifies an entity/a record
- a Foreign Key is a copy of the Primary Key of one entity class/table that appears as an attribute/field in another entity class/table and helps define the relationship between entities/records in the two entity classes/tables
- these keys link together the related entity sets/tables in a relational database
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