Objects are very often found in relationships called hierarchical. Usually we are talking about relationships such as "part-whole", the relationship of subordination or species relationship. Objects that are in such a relationship form a tree called a directed graph, that is, having only one vertex that is not subordinate to any other vertex. This vertex is called the root of the tree. The interaction between the other vertices is implemented as follows: any other vertex obeys only one vertex located above it. In conceptual terms, a hierarchical data model is understood as a collection of record types that are linked by types of relationships into one single or several separate trees. In this model, all types of relationships are formed in a one-to-several relationship,and on the graph it is depicted by arrows. It turns out that the hierarchical data model has this type of relationship between objects as in the family tree, but there is one exception: for each subordinate object there is only one main object. That is, in suchdata model, only two types of communication between database objects are allowed: one-to-one or one-to-many. The hierarchical database is inherently a navigation database, that is, access is possible exclusively through previously defined connections.
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From this material you can get some idea of what a hierarchical data structure is.