Department «digital economy and information technology» subject "database"

Hierarchical data model (HDM)
The hierarchical model allows you to build a database with a hierarchical tree structure. The structure of the IMD is described in terms similar to those of the network data model. A group in the IMD is usually calledsegment.IMD is based on the concept of a tree.
Tree -is a connected undirected graph that does not contain cycles. When working with a tree, they select a specific vertex, define it as the root of the tree and consider it separately - not a single edge goes into this vertex. In this case, the tree becomes oriented, the orientation is determined from the root. A tree as a directed graph is defined as follows:-there is a single special vertex, called the root, into which no edge enters;-all other vertices have only one edge entering, and an arbitrary number of edges emanating;-the graph does not contain cycles.
Hierarchical data model (HDM)
Hierarchical data models use a tree structure orientation from root to leaves. A graphical diagram of a conceptual database schema is calleddefinition tree. An example of a hierarchical database is given below.
Hierarchical data model (HDM)
The main disadvantage of IMD is data duplication. It is caused by the fact that each entity (attribute) can only relate to one parent entity. For example, if the database stores data about employees’ children, and the company also employsfather, Andmotherchild, theninformation about this child must be stored twice. A similar situation arisesif you need to reflect a many-to-many relationship in the database.Data duplication can cause a violation of the logical integrity of the database when changes are made to this data.
Relational Data Model (RDM)
The relational data model was proposed in 1970 by mathematician Edgar Codd (Codd E.F.). RMD is the most widely used data model and the only one of the three main data models for which a theoretical basis has been developed using set theory.
The basic structure of RMD is the relation, based onCartesian product of domains.A domain is a set of values ​​that can take ondata element(for example, many integers, many dates, many combinations of characters of lengthNand so on.). A domain can be specified by listing elements, specifying a range of values, a function, etc.