The hierarchical model was developed in the 1960s to manage large amounts of data for complex manufacturing projects such as the Apollo rocket that landed on the moon in 1969. Its basic logical structure is represented by an upside-down tree. The hierarchical structure contains levels, or segments. A segment is the equivalent of a file system’s record type. Within the hierarchy, a higher layer is perceived as the parent of the segment directly beneath it, which is called the child. The hierarchical model depicts a set of one-to-many (1:M) relationships between a parent and its children segments. (Each parent can have many children, but each child has only one parent.)
The network model was created to represent complex data relationships more effectively than the hierarchical model, to improve database performance, and to impose a database standard. In the network model, the user perceives the network database as a collection of records in 1:M relationships. However, unlike the hierarchical model, the network model allows a record to have more than one parent. While the network database model is generally not used today, the definitions of standard database concepts that emerged with the network model are still used by modern data models. Some important concepts that were defined at this time are:
- The schema, which is the conceptual organization of the entire database as viewed by the database administrator.
- The subschema, which defines the portion of the database “seen” by the application programs that actually produce the desired information from the data contained within the database
- A data management language (DML), which defines the environment in which data can be managed and to work with the data in the database
- A schema data definition language (DDL), which enables the database administrator to define the schema components