What Is a Database?
A database is a collection of records and files that are organized for a particular purpose. On
our computer system, you might keep the names and addresses of all our friends or customers.
Perhaps you collect all the letters you write and organize them by recipient. You might have
another set of files in which you keep all our financial data—accounts payable and accounts
receivable or our checkbook entries and balances. The word processor documents that you organize
by topic are, in the broadest sense, one type of database. The spreadsheet files that you
organize according to their uses are another type of database. Shortcuts to all our programs in
our Windows Start menu are a kind of database. Internet shortcuts organized in our Favorites
folder are a database.
If you're very organized, you can probably manage several hundred spreadsheets or shortcuts by
using folders and subfolders. When you do this, you're the database manager. But what do you do
when the problems you're trying to solve get too big? How can you easily collect information
about all customers and their orders when the data might be stored in several document and
spreadsheet files? How can you maintain links between the files when you enter new information?
How do you ensure that data is being entered correctly? What if you need to share our
information with many people but don't want two people to try updating the same data at the same
time? Faced with these challenges, you need a database management system (DBMS).
Relational Databases
Nearly all modern database management systems store and handle information using the relational
database management model. The term relational stems from the fact that each record in the
database contains information related to a single subject and only that subject. If you study the
relational database management model, you'll find the term relation applied to a set of rows
about a single subject. Also, data about two classes of information (such as customers and
orders) can be manipulated as a single entity based on related data values. For example, it would
be redundant to store customer name and address information with every order that the customer
places. In a relational database system, the information about orders contains a field that
stores data, such as a customer number, which can be used to connect each order with the
appropriate customer information.
In a relational database management system, sometimes called an RDBMS, the system manages all
data in tables. Tables store information about a subject (such as customers or products) and have
columns that contain the different kinds of information about the subject (for example,
customers' addresses or book titles) and rows that describe all the attributes of a single
instance of the subject (for example, data on a specific customer or book). Even when you query
the database (fetch information from one or more tables), the result is always something that
looks like another table.
You can also join information on related values from multiple tables or queries. For example, you
can join author information with book information to find out which authors wrote which books.
You can join employee information with contract information to find out which salesperson should
receive a commission.
Relational Database Terminology
Relation. Information about a single subject such as customers, orders, golfers, golfer scores,
or entertainment groups. A relation is usually stored as a table in a relational database
management system.
Attribute. A specific piece of information about a subject, such as the address for a customer or
the dollar amount of a contract. An attribute is normally stored as a data column, or field, in a
table.
Relationship. The way information in one relation is related to information in another relation.
For example, customers have a one-to-many relationship with orders because one customer can place
many orders, but any order belongs to only one customer. Entertainment groups might have a
many-to-many relationship with night clubs because each group is interested in working for
multiple clubs, and each club will book several groups over time.
Join. The process of linking tables or queries on tables via their related data values. For
example, customers might be joined to orders by matching customer ID in a customers table and an
orders table.
Database Capabilities
An RDBMS gives we complete control over how we define our data, work with it, and share it with others. The system also provides sophisticated features that make it easy to catalog and manage large amounts of data in many tables. An RDBMS has three main types of capabilities:
data definition, data manipulation, and data control.
Data definition. we can define what data will be stored in our database, the type of data (for example, numbers or characters), and how the data is related. In some cases, we can also define how the data should be formatted and how it should be validated.
Data manipulation. we can work with the data in many ways. we can select which data fields we want, filter the data, and sort it. we can join data with related information and summarize (total) the data. we can select a set of information and ask the RDBMS to update it, delete it, copy it to another table, or create a new table containing the data.
Data control. we can define who is allowed to read, update, or insert data. In many cases, we can also define how data can be shared and updated by multiple users.
All this functionality is contained in the powerful features of Microsoft Access. Let's take a look at how Access implements these capabilities and compare them to what we can do with spreadsheet or word processing programs.
|
No responses found. Be the first to respond and make money from revenue sharing program.
|