In the context of database management systems (DBMS), a data model can be described as a way of organizing and representing data within the system. There are various types of data models such as the relational data model, the entity-relationship data model, the object-based data model, and the semistructured data model.
Regardless of the type of data model being used, there are two important concepts that must be understood: the data model schema and the data model instance.
Data Model Schema: The data model schema can be thought of as a blueprint or a plan for how the data will be structured within the database. It defines the various tables, their attributes, and the relationships between them. The data model schema is created during the database design phase and acts as a guide for the implementation of the database.
For example, in the relational data model, the schema would define the tables, their attributes, and the relationships between them using concepts such as primary and foreign keys. In the entity-relationship data model, the schema would define the entities, their attributes, and the relationships between them using concepts such as cardinality and participation.
Data Model Instance: The data model instance can be thought of as a snapshot of the data at a particular point in time. It is the actual data that is stored within the database, based on the structure defined by the data model schema. The data model instance is constantly changing as data is added, deleted, or updated within the database.
For example, in the relational data model, the instance would consist of the actual data stored in the tables, such as customer information or sales transactions. In the entity-relationship data model, the instance would consist of the actual instances of the entities and their attributes, such as a specific customer or a specific product.
In the database, actual data changes quite frequently. For example, in the given figure, the database changes whenever we add a new grade or add a student. The data at a particular moment of time is called the instance of the database.
In summary, the data model schema and instance are important concepts in understanding how data is organized and represented within a database management system. The schema acts as a guide for how the data is structured, while the instance represents the actual data stored within the database at a particular point in time.