What are the three data models of the database?
Hierarchical model
The hierarchical model is the earliest model used in database systems. Its data structure is a "directed tree" . The root node is at the top, the highest level, and the child nodes are at the bottom, arranged layer by layer. The characteristics of the hierarchical model are:
1. There is one and only one root node;
2. Other nodes have one and only one parent node.
Network model
The network model represents the relationship between entities in a network structure. Each node in the network represents a record type, and the relationship is implemented using link pointers. The network model can represent the connection of multiple subordinate relationships, and can also represent the cross-relationship between data, that is, the horizontal relationship and vertical relationship between data. It is an extension of the hierarchical model. The network model can conveniently represent various types of connections, but its structure is complex and the implemented algorithm is difficult to standardize. Its characteristics are:
1. A node is allowed to have more than one parent node;
2. There can be more than one node without a parent node.
Relationship model
The relational model uses a two-dimensional table structure to represent the relationship between entities. It is based on the theory of relational mathematics. The data structure of the relational model is a collection of "two-dimensional table frames". Each two-dimensional table can also be called a relationship. In the relational model, the objects and results of operations are two-dimensional tables. The relational model is currently the most popular database model. A database management system that supports the relational model is called a relational database management system, and Access is a relational database management system.
Features:
1. Consistency of description, not only uses relationships to describe the entities themselves, but also uses relationships to describe the connections between entities;
2. Can be expressed directly Many-to-many relationship;
3. The relationship must be a standardized relationship, that is, each attribute is an inseparable data item, and there is no table in the table;
4. The relationship model is built on It is based on mathematical concepts and has a strong theoretical basis.
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