Microsoft Office Tutorials and References
In Depth Information
A relational database stores its data in tables. A table is a two-dimensional structure made
up of rows and columns. The terms table, record (row), and field (column) are the popular
names for the more formal terms relation (table), tuple (row), and attribute (column), as
shown in Figure A-1.
A table (relation) consisting of records and fields
re c ords (rows
The Employee table shown in Figure A-1 is an example of a relational database table, a
two-dimensional structure with the following characteristics:
• Each row is unique. Because no two rows are the same, you can easily locate and update
specific data. For example, you can locate the row for EmployeeNum 8005 and change
the FirstName value, Pat, the LastName value, Amidon, or the DeptNum value, 27.
• The order of the rows is unimportant. You can add or view rows in any order. For exam-
ple, you can view the rows in LastName order instead of EmployeeNum order.
• Each table entry contains a single value. At the intersection of each row and column, you
cannot have more than one value. For example, each row in Figure A-1 contains one
EmployeeNum value, one FirstName value, one LastName value, and one DeptNum value.
• The order of the columns is unimportant. You can add or view columns in any order.
• Each column has a unique name called the field name . The field name allows you to
access a specific column without needing to know its position within the table.
• The entries in a column are from the same domain. A domain is a set of values from which
one or more columns (fields) draw their actual values. A domain can be broad, such as “all
legitimate last names of people” for the LastName column, or narrow, such as “24, 27, or
31” for the DeptNum column. The domain of “all legitimate dates” could be shared by the
BirthDate, StartDate, and TerminationDate columns in a company’s employee table.
• Each row in a table describes, or shows the characteristics of, an entity. An entity is a
person, place, object, event, or idea for which you want to store and process data. For
example, EmployeeNum, FirstName, LastName, and DeptNum are characteristics of the
employees of a company. The Employee table represents all the employee entities and
their characteristics. That is, each row of the Employee table describes a different
employee of the company using the characteristics of EmployeeNum, FirstName,
LastName, and DeptNum. The Employee table includes only characteristics of employ-
ees. Other tables would exist for the company’s other entities. For example, a
Department table would describe the company’s departments and a Position table
would describe the company’s job positions.