There are four structural types of database management systems: hierarchical, network, relational, and object-oriented.
1) Hierarchical Database: A hierarchical database model is a data model in which the data is organized into a tree-like structure. The structure allows representing information using parent/child relationships: each parent can have many children, but each child has only one parent (also known as a 1-to-many relationship). All attributes of a specific record are listed under an entity type. A hierarchical database is organized in pyramid fashion, like the branches of a tree extending downwards. Related fields or records are grouped together so that there are higher-level records and lower-level records.
The advantage of hierarchical databases is that they can be accessed and updated rapidly because the tree-like structure and the relationships between records are defined in advance. However, this feature is a two-edged sword. The disadvantage of this type of database structure is that each child in the tree may have only one parent, and relationships or linkages between children are not permitted, even if they make sense from a logical standpoint. Hierarchical databases are so rigid in their design that adding a new field or record requires that the entire database be redefined.
In a database an entity type is the equivalent of a table. Each individual record is represented as a row, and each attribute as a column. Entity types are related to each other using 1:N mappings, also known as one-to-many relationships. This model is recognized as the first database model created by IBM in the 1960s. Currently the most widely used hierarchical databases are IMS developed by IBM.
2) Network database: The network model is a database model conceived as a flexible way of representing objects and their relationships. Its distinguishing feature is that the schema, viewed as a graph in which object types are nodes and relationship types are arcs, is not restricted to being a hierarchy or lattice.
The network model’s original inventor was Charles Bachman, and it was developed into a standard specification published in 1969 by the CODASYL Consortium.
Like hierarchical databases, network databases are principally used on mainframe computers. Since more connections can be made between different types of data, network databases are considered more flexible. However, two limitations must be considered when using this kind of database. Similar to hierarchical databases, network databases must be defined in advance. There is also a limit to the number of connections that can be made between records.
3) Relational Database: Relational databases connect data in different files by using common data elements or a key field. Data in relational databases is stored in different tables, each having a key field that uniquely identifies each row. Relational databases are more flexible than either the hierarchical or network database structures. In relational databases, tables or files filled with data are called relations, tuples designates a row or record, and columns are referred to as attributes or fields.
Relational databases work on the principle that each table has a key field that uniquely identifies each row, and that these key fields can be used to connect one table of data to another. Thus, one table might have a row consisting of a customer account number as the key field along with address and telephone number. The customer account number in this table could be linked to another table of data that also includes customer account number (a key field), but in this case, contains information about product returns, including an item number (another key field). This key field can be linked to another table that contains item numbers and other product information such as production location, color, quality control person, and other data. Therefore, using this database, customer information can be linked to specific product information.
The relational database has become quite popular for two major reasons. First, relational databases can be used with little or no training. Second, database entries can be modified without redefining the entire structure. The downside of using a relational database is that searching for data can take more time than if other methods are used.
4) Object-oriented Databases (OODBMS): An object-oriented database can be used to store data from a variety of media sources, such as photographs and text, and produce work, as output, in a multimedia format. Object-oriented databases use small, reusable chunks of software called objects. The objects themselves are stored in the object-oriented database.
Each object consists of two elements:
- a piece of data (e.g ., sound, video, text, or graphics),
- the instructions, or software programs called methods, for what to do with the data.
The instructions contained within the object are used to do something with the data in the object. For example, test scores would be within the object as would the instructions for calculating average test score.
Object-oriented databases have two disadvantages. First, they are more costly to develop. Second, most organizations are reluctant to abandon or convert from those databases that they have already invested money in developing and implementing. However, the benefits to object-oriented databases are compelling. The ability to mix and match reusable objects provides incredible multimedia capability. Healthcare organizations, for example, can store, track, and recall CAT scans, X-rays, electrocardiograms and many other forms of crucial data.