Databases Reference
In-Depth Information
No two rows of the table are identical.
•
Let us make some remarks about the concept of a table:
A table may (but is not required to) have a name, such as BOOKS, which is
intended to convey the meaning of the table as a whole.
•
The number of rows of the table is called the
size
of the table, and the number of
columns is called the
degree
of the table. For example, the BOOKS table shown
in Table 3-1 has size 14 and degree 3. The attribute names are ISBN, Title, and
Price.
•
As mentioned earlier, to emphasize the attributes of a table, it is common to
denote a table by writing T(A
1,
...,A
n
); for example, we denote the BOOKS table
by:
•
BOOKS(ISBN,Title,Price)
The order of the rows of a table is not important, and so two tables that differ only
in the order of their rows are thought of as being the same table. Similarly, the
order of the columns of a table is not important as long as the headings are
thought of as part of their respective columns. In other words, we may feel free to
reorder the columns of a table, as long as we keep the headings with their
respective columns.
•
Finally, there is no requirement that the domains of different columns be different.
(For example, it is possible for two columns in a single table to use the domain of
integers.) However, there is a requirement that the attribute names of different
columns be different. Think of the potential confusion that would otherwise
ensue, in view of the fact that we may rearrange the columns of a table!
•
Now that we have defined the concept of a table, we can say that it is common to define a
relational database as a finite collection of tables. However, this definition belies the fact
that the tables also model the relationships between the entity classes, as we will see.
3.2 A Short Glossary
To help keep the various database terms clear, let us collect their definitions in one place:
Entity
An object about which the database is designed to store information. Example: a
book; that is, an ISBN, a title, and a price, as in:
0-12-333433-3, On Liberty, $25.00
Attribute
A property that (partially or completely) describes an entity. Example: title.
