The first model that was developed was called the conceptual data model and it was usually represented by an entity relationship diagram (ERD). The purpose of the ERD was to provide an abstraction that represented the data requirements of the organization. Most people with any experience of database design would be familiar with the ERD approach to designing databases.
One major characteristic of the conceptual model is that it ought to be able to be implemented using any type of database. In the 1970s, the relational database was not the most widely used type of DBMS. In those days, the databases tended to be:
Hierarchical databases
Network databases
Once the DBMS had been chosen, then a logical model would be produced. The logical model was normally expressed as a schema in textual form. So, for instance, where the solution was to be implemented using a relational database, a relational schema would be produced. This consisted of a set of relations, the relationships expressed as foreign key constraints, and a set of domains from which the attributes of the relations would draw their values.
The physical data model consisted of the data definition language (DDL) statements that were needed to actually build, in a relational environment, the tables, indexes, and constraints. This is sometimes referred to as the implementation model.
One of the strengths of the trilogy is that decisions relating to the logical and physical design of the database could be taken and implemented without affecting the abstract model that reflected the business requirements.
The first model that was developed was called the conceptual data model and it was usually represented by an entity relationship diagram (ERD). The purpose of the ERD was to provide an abstraction that represented the data requirements of the organization. Most people with any experience of database design would be familiar with the ERD approach to designing databases.
One major characteristic of the conceptual model is that it ought to be able to be implemented using any type of database. In the 1970s, the relational database was not the most widely used type of DBMS. In those days, the databases tended to be:
Once the DBMS had been chosen, then a logical model would be produced. The logical model was normally expressed as a schema in textual form. So, for instance, where the solution was to be implemented using a relational database, a relational schema would be produced. This consisted of a set of relations, the relationships expressed as foreign key constraints, and a set of domains from which the attributes of the relations would draw their values.
The physical data model consisted of the data definition language (DDL) statements that were needed to actually build, in a relational environment, the tables, indexes, and constraints. This is sometimes referred to as the implementation model.
One of the strengths of the trilogy is that decisions relating to the logical and physical design of the database could be taken and implemented without affecting the abstract model that reflected the business requirements.