Properties of Relational Tables
Relational tables have six properties:
1. Values are atomic.
2. Column values are of the same kind.
3. Each row is unique.
4. The sequence of columns is insignificant.
5. The sequence of rows is insignificant.
6. Each column must have a unique name.
This property implies that columns in a relational table
are not repeating group or arrays. Such tables are referred to as being in the
"first normal form" (1NF). The atomic value property of relational
tables is important because it is one of the cornerstones of the relational
model.
The key benefit of the one value property is that it
simplifies data manipulation logic.
Column Values Are of the Same Kind
In relational terms this means that all values in a column
come from the same domain. A domain is a set of values which a column may have.
For example, a Monthly_Salary column contains only specific monthly salaries.
It never contains other information such as comments, status flags, or even
weekly salary.
This property simplifies data access because developers
and users can be certain of the type of data contained in a given column. It
also simplifies data validation. Because all values are from the same domain,
the domain can be defined and enforced with the Data Definition Language (DDL)
of the database software.
This property ensures that no two rows in a relational
table are identical; there is at least one column, or set of columns, the
values of which uniquely identify each row in the table. Such columns are
called primary keys and are discussed in more detail in Relationships
and Keys.
This property guarantees that every row in a relational
table is meaningful and that a specific row can be identified by specifying the
primary key value.
The Sequence of Columns is Insignificant
This property states that the ordering of the columns in
the relational table has no meaning. Columns can be retrieved in any order and
in various sequences. The benefit of this property is that it enables many
users to share the same table without concern of how the table is organized. It
also permits the physical structure of the database to change without affecting
the relational tables.
The Sequence of Rows is Insignificant
This property is analogous the one above but applies to
rows instead of columns. The main benefit is that the rows of a relational
table can be retrieved in different order and sequences. Adding information to
a relational table is simplified and does not affect existing queries.
Because the sequence of columns is insignificant, columns
must be referenced by name and not by position. In general, a column name need
not be unique within an entire database but only within the table to which it
belongs.