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Database Concepts

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Definition of a Database An organized Collection Of related records

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Database Management System Software that helps you organize

data in a way that allows fast and easy access to data

Data in a database is stored differently than in other applications: In those forms, data is entered and

used in the same form as it resides on disk

In a database, data can reside on the disk in ways unknown to a user

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Advantages of Databases Reduced Redundancy: data which might be stored more

than once in separate files, are stored in only one place or file in a database.

Integrated Data: rather than being in separate and independent files, data is integrated because any item of data can be used to satisfy an inquiry or report.

Data Integrity: that the files are accurate and up-to-date Edit checks on various fields to insure that the data keyed

in at least conforms to standards we have established. Because one item of data should not be stored in two

different files of a database, it prevents data being stored correctly in one file and incorrectly in another file.

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Database Models The way a database organizes data

depends on the type, or model, of the database

Three types: Hierarchical Network Relational – organizes data in a table

form consisting of related rows and columns

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Fields, Records, and Keys - 1 Each “box” in a table contains a data item Each column is a field, which is a type of

data – every data item in a field has similar characteristics and meanings.

Full set of data in any given row is a record.

A collection of related records makes a file, or, in database nomenclature, a table

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Fields, Records, and Keys - 2 The file or table is the

underlying basis of all relational databases.

Each table in a database must have a unique identifier, or key

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Database Power The relationships between each file in the

database How these relationships allow you to

connect one file to another Connected by a field in each table, called a

common field. They may not have the same name, but will have the same characteristics and meaning

Common field is usually the key to one table, and an embedded field in another table

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Three Types of Relationships not Discussed in the Text One to many:

Most common Example: salesperson to customer – one

salesperson may have more than one customer One to one:

Example: salesperson to company car Many to many:

Example: inventory items to customer orders. We won’t create this kind of relationship in class

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Parent-Child & Enforcing Referential Integrity In a one to many relationship, the one

table is defined as the “parent” and the many table is defined as the “child”

Referential Integrity We must define which table is the parent,

and which is the child We must define the common field While the common fields in each table do

not have to have the same name, they must have the same characteristics

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What Enforcing Referential Integrity Means We cannot add a new child record if its

common field references a non-existent parent

We cannot delete a parent which has existing children

If a salesman is the parent, and the customers are his/her children, then we cannot delete the parent until the children are reassigned to a new parent

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Example of Related TablesSP ID Last Name First Name Office Hire Date Salary

12 Davis James Savannah 12/01/95 $45,000

15 Jones Susan Savannah 10/14/94 $51,500

16 Wilson William Augusta 07/08/94 $53,400

CUST ID Customer Name City State Credit Limit Salesperson

1004 XYZ Co. Vidalia GA $55,000 12

1007 PDQ Distributing Garden City GA $150,000 12

1009 ABC, Inc. Savannah GA $200,000 15

1011 SC Wholesale N. Augusta SC $85,000 16

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The Database Management Systems (DBMS) Process

Describe the data

Enter the data

Review the data and edit until accurate

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Queries - 1 Another way of viewing data in a table May be used to view data in one table,

or any multiple number of tables joined by their relationships

Can query a table from any field in the table (integrated data)

Can sort the data either ascending (A to Z) or descending (Z to A) from any field in the table

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Queries - 2 The power of a query are the two following points:

A query allows us to view only the fields we wish to see. If a table contains as many as 50 fields, we can view only those we want to.

A query allows us to view only the records (rows) we wish to see, by setting the criteria for viewing the data

We can, for example, view only the customers who are from GA, or also just from GA with a specified sales amount

Can also join two tables in a query, and further specify the data we wish to see via the criteria selection.

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Queries - 3 Created either by

SQL - Structured Query Language Entered directly by user Included in programs

Or by QBE – Query by Example

Graphical interface to specify criteria This is what we will do in MS Access

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Report Formatted presentation of data from the

database Normally printed Designed either by using a report

generator (a wizard in Access, which is what we will do) or from “scratch” in Access “Design View”

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Concurrency Control Databases are used concurrently

by many users Problem if several users attempt to

update the same record at the same time

Record locking First user requests record Others are locked out for update