Thursday, April 23, 2009

Relational Structuring.

 In relational structuring, all data and relationships are represented in a flat, two-dimensional table called a relation. A relation is equivalent to a file, where each line represents a record. Note that all the entries in e column are of the same kind. Furthermore, each column has a unique name. Finally, no two rows in the table are identical. A row is referred to as tuple (rhymes with "couple”).

A relational DBMS has several features:
1. It allows the user to update (add, modify, or delete) the table's contents.
Any position can be changed.

2. It provides inquiry capabilities against a label. Using our example, inquiry might     be:" How many years has Boynton been with the firm?” The response is "6."

  3. Two or more tables can be merged to form one relation. Unlike hierarchical or network structuring where all relationships are predefined, a relational DBMS develops new relations on user commands.

            To illustrate, suppose a relational DBMS maintains two relations: the EMPLOYEE relation and the EMPLOYEE EDUCATION relation .A query about employees with more than two years the firm and an MBA puts the relational DBMS through the following routine:

1. A relationship is implied between the EMPLOYEE relation and EMPLQYEE EDUCATION relation.

   2. A temporary table of employees who have been in the firm more t two years is obtained from the EMPLOYEE relation and placed in file. This is called a temporary relation and will be deleted after the user has been satisfied.

   3. The information in the temporary table is taken along with the EM­ PLOYEE EDUCATION relation to determine which employee who has been in the firm more than two years has an MBA. This results in a second temporary relation. In our example, zmud and Boynton are the only employees. The temporary relation is also deleted after the user query has been satisfied.

A relational structure is simpler to construct than a hierarchical or a network structure. It may be inefficient, though, since relational DBMS ponds to queries by an exhaustive review of the relations involved.

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