To model the entities effectively, representing regular users, premium members, and administrators as separate entities using Generalization and Specialization is suitable. Each entity can have its own specific attributes and behaviors, allowing for flexible and clear modeling of customer types.

In UML, the Class diagram represents the static view of a system. It illustrates the structure of the system by showing classes, their attributes, and the relationships between classes. This diagram is essential for understanding the organization of objects in the system.

Having a mandatory relationship ensures that all related records must be present, promoting data integrity. This constraint guarantees that every record in the "many" side is linked to a corresponding record in the "one" side, preventing orphaned or incomplete relationships.

A foreign key constraint in a database table is used to define a link between a column in one table and a column in another table. This establishes referential integrity, preventing inconsistent data by ensuring that values in the foreign key column match those in the referenced primary key column.

Reverse Engineering helps in understanding the existing database schema by documenting its structure and relationships. This facilitates database documentation, making it easier for developers and database administrators to comprehend and work with the database.

In an ERD, an entity represents a real-world object or concept, such as a person, place, thing, or event, about which data is stored in a database. Entities are typically nouns and serve as the foundation for the structure of the database. Entities are depicted as rectangles in an ERD.

The fundamental data structure used in a Key-Value Store is a Hash Table. Hash Tables provide efficient key-based access to values by mapping keys to specific locations in the table. This allows for quick retrieval of values based on their associated keys.

In specialization, "subtypes" refer to the specialized entities that are derived from a more generalized entity. These subtypes inherit the attributes and relationships of the parent entity but may also have unique characteristics specific to their subtype.

In a graph database, a node represents an entity or data point. Nodes are used to store and represent individual entities, and relationships between nodes express the connections or associations between these entities. This allows for a flexible and efficient representation of complex relationships in data.

In this scenario, denormalization can be applied by creating indexed views for financial reports. This involves introducing redundancy strategically to optimize reporting queries, as the indexed views store precomputed results for faster retrieval.

The process of creating specialized entities from a general entity is called "Specialization." It involves identifying and defining more specific entities (subtypes) based on common characteristics of a general entity (supertype).

UML focuses on Object-oriented modeling, emphasizing the representation of objects, classes, and their relationships. On the other hand, SQL focuses on Relational modeling, dealing with tables, data integrity, and relationships between tables in a relational database.