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The Relational Database Model

الكلية كلية تكنولوجيا المعلومات     القسم قسم البرامجيات     المرحلة 2
أستاذ المادة مازن كاظم حميد المنصوري       05/01/2016 09:01:20
The Relational Database Model

The logical view of the relational database is facilitated by the creation of data relationships based on a logical construct known as a relation. Because a relation is a mathematical construct, end-users find it much easier to think of a relation as a table.
A table is perceived as a two-dimensional structure composed of rows and columns.

Characteristics of a Relational Table:

- A table is perceived as a two-dimensional structure composed of rows and columns.
- Each table row(tuple) represents a single entity occurrence within the entity set.
- Each table column represents attribute, and each column has a distinct name.
- Each row/column intersection represents a single data value.
- All values in a column must conform to the same data format.
- Each column has a specific range of values known as the attribute
domain.
- The order of the rows and columns is immaterial to the DBMS.
- Each table must have an attribute or a combination of attributes that uniquely identities each row.

Keys

In the relational model, keys are important because they are used to ensure that each row in a table is uniquely identifiable. They are also used to establish relationships among tables and to ensure the integrity of the data. Therefore, a proper understanding of the concept and use of keys in the relational model is very important. A key consists of one or more attributes that determine other attributes. For example, an invoice number identifies all of the invoice attributes, such as the invoice data and the customer name
The keys role is based on a concept known as determination, when the statement" A determines B" indicates that if you know the value of attribute A, you can loop up (determine) the value of attribute B, for example as in table student

Num LName FName Init Phone Class Hrs
232 smith William C 2267 So 42
233 Bowser Anne M 2255 Jr 81
234 Katinga John P 2267 So 36
235 Robertson John K 2277 Jr 66









NUM LNAME
NUM LNAME, FNAME, INIT


The principle of determination is very important because it is used in the definition of a central relational database concept known as functional dependence.

The attribute B is functionally dependent on the attribute A if each value in column A determines one and only one value in column B.


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