Database Reference
In-Depth Information
You access information manually by opening a file cabinet, taking out a file folder, and finding the
correct piece of paper. Customers fill out paper forms for input, perhaps by using a keyboard to input
information that is printed on forms. You find information by manually sorting the papers or by copy-
ing information from many papers to another piece of paper (or even into an Excel spreadsheet). You
may use a spreadsheet or calculator to analyze the data or display it in new and interesting ways.
Tables
Databases store information in carefully defined structures called
tables.
A table is just a container for
raw information (called
data
), similar to a folder in a manual filing system. Each table in a database
contains information about a single entity, such as a person or product, and the data in the table is
organized into rows and columns. A relational database system stores data in related tables. For
example, a table containing employee data (names and addresses) may be related to a table contain-
ing payroll information (pay date, pay amount, and check number).
In database-speak, a table is an object. As you design and work with databases, it's
important to think of each table as a unique entity and consider how each table relates
to the other objects in the database.
Note
In most database systems, you can view the contents of a table in a spreadsheet-like form, called a
datasheet
, comprising rows and columns (known as
records
and
fields
, respectively — see the follow-
ing section, “Records, fields, and values”). Although a datasheet and a spreadsheet are superficially
similar, a datasheet is a very different type of object. You typically cannot make changes or add calcu-
lations directly within a table. Your interaction with tables primarily comes in the form of queries or
views (see the later section, “Queries”).
Records, fields, and values
A database table is divided into rows (called
records
) and columns (called
fields
), with the first row
(the heading at the top of each column) containing the names of the fields in the database.
Each row is a single record containing fields that are related to that record. In a manual system, the
rows are individual forms (sheets of paper), and the fields are equivalent to the blank areas on a
printed form that you fill in.
Each column is a field that includes many properties that specify the type of data contained within
the field, and how the database should handle the field's data. These properties include the name of
the field (for example, CompanyName) and the type of data in the field (for example Text). A field may
include other properties as well. For example, a field's Size property tells the database the maximum
number of characters allowed for the address.
At the intersection of a record and a field is a
value
— the actual data element. For example, if you
have a field called CompanyName, a company name entered into that field would represent one data
value.
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