Relational Databases
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
[top]
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
[top]
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
[top]
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.
To make the distinctions clear, we will use a color schema in which we use
terms relevant to entity relationship models
and their correlates in Access
and, when appropriate, their correlates in Excel
In database language, objects are entities;
in Access, entities are records
in Excel, entities are rows
| type | ⇒⇒⇒⇒⇒⇒ | ⇓⇓⇓⇓⇓⇓ |
|---|---|---|
| ERModel entity set | composed of entities | which have attributes |
| Access tables | composed of records | which have fields |
| Excel worksheets | composed of rows | which have column cells |
An entity class [like books] is an abstract definition of something
an entity (like a textbook for INLS161-002) 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
attributes/fields include information we want in the database
attributes/fields help to uniquely identify individual entities within a class
attributes/fields 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
[top]