2.1.3 Relational databases

    In the relational model, entities and the connections between entities are represented by relationships. For example, the one-to-many relationship between the tutor entity, the graduate student entity, and the tutor and the graduate student can be represented by a relationship respectively. In a given application domain, the set of all entity-to-entity relationships constitutes a relational database.
    Relational databases also have types and values. A relational database schema, also known as a relational database schema , is a description of a relational database. A relational database schema includes:
    - Definitions of fields;
    - Relational schemas defined on these fields.
    The value of a relational database is a collection of relations corresponding to these relational schemas at a certain time, which is usually called a relational database.