Hierarchical database model

A hierarchical database model is the oldest database model, it is the real world by a hierarchical tree structure. Each set (record ) has exactly one predecessor, with the exception of exactly one sentence, namely the root of the resulting tree.

The data is stored in a series of records with which different fields are linked. The instances of a particular data set are summarized as the record image. These record images are comparable to the tables of a relational database.

Links between the Datensatzabbildern be realized in hierarchical databases as parent-child relationships ( parent-child Relationships, PCR), which are displayed in a tree structure. The disadvantage of hierarchical databases is that they can only deal with such a tree. Links between different trees or across multiple levels within a tree are not possible.

With the two structural elements ( record types and PCR types) can be the following minimum conditions to a hierarchical database model set:

• A record type must represent the root element, and therefore does not occur as a "Child " on in a PCR relationship.
• Any other type of record occurs exactly once as a " Child"
• A record type that does not occur as a " parent" in a PCR types is called "leaf"

N relationships represent (see chart): this tree is only 1:1 and 1 can. The often necessary n: m - relationships can be achieved firstly through redundancies, but better about virtual Parent-Child Relationships ( VPCR ).

The hierarchical model has today been largely replaced in the field of database systems of other database models.

File systems of many operating systems are " approximately" hierarchical databases: files correspond sheets, directories / folders corresponding to the nodes with children. The similarity is incomplete because directories without files still no leaves ( flat files ) are in (real example, they contain references to themselves and the parent node, namely with "." And ".." ), and because the use of soft links or hard links will not get the tree structure.

A renaissance hierarchical data storage using XML. Also various legacy applications with banks and insurance companies use hierarchical databases today. The most famous hierarchically organized database system is IMS / DB from IBM.