Logical Data Model
The logical data model brings data modeling closer to implementation. Here the type of database system to be implemented has a bearing on the construction of the data model. If you are implementing a relational database system, the logical data model takes one specific form. If it is going to be a hierarchical or network database system, the form and composition of the logical data model differs. Nevertheless, still considerations of specific DBMS (particular database software) and hardware arc kept out.
If you are implementing a relational database system, your logical model consists of two-dimensional tables called relations with columns and rows. In the relational convention, data content is perceived in the form of tables or relations. Relationships among the tables are established and indicated through logical links using foreign key columns. More details on foreign keys will follow later on.
The key task in building a logical data model is to precisely define the set of objects of interest and to identify the relationships between them. Some examples of objects you might consider are streets, parcels, owners, and buildings. Some-examples of their relationships are "located at" "owned by," and "Is pan of”. Once you have an initial logical data model, you can validate it against the user's requirements for entering, updating, and accessing data and by testing it against the organization's practices and procedures (or business rules). It is especially important to involve representatives from each prospective user group. A logical data model built for a subset of users is guaranteed to have deficiencies for overlooked users.
Representing logical Data Models
In the past, logical data models were often drawn in what are known as entity-relationship diagrams. A number of leading object-oriented modelers put forward various design methodologies and diagram notations. These methodologies emphasized different aspects such as data flow or use-case scenarios, but a problem with entity-relationship diagrams is that their appearance varied with the design methodology. More recently, most object-oriented modelers have adopted the Unified Modeling language (UML), which is a standard notation for expressing object models and is endorsed by leading software and database companies. It is important to note that UML is not a design methodology, but rather a diagrammatic notation. With UML, You can adopt the object-oriented design methodology of your choosing and express the model in a standard way.