Answer to Question 1
The most successful DBMS model in present day information systems in the relational model that was introduced before object-oriented technologies. A relational DBMS, by hiding the physical structure of data, presents a logical view of the database organization and provides the basis for using a high-level language for database management.
In the relational model, a table is the basic organizational unit for data. A table is a rectangular and flat arrangement of data, composed of attributes (or columns) and entities (or rows). The intersection of a column and a row is known as a cell. A primary key is an attribute or a set of attributes whose values uniquely identify a row. In addition, an entity may have other unique identifiers known as alternate or candidate keys.
Answer to Question 2
A structural model is distinguished from another structural model by the nature of its building blocks (or units) and how the units are connected. The structural model of information systems is composed of classes and their interrelationships. At runtime, when the information system is actually created, classes are instantiated into objects that function as the units of the information system. In other words, classes act both as units of modeling and as templates for units of the information system.
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