Database Models
● Hierarchical
● Network
● Relational
Hierarchical Model:
This model was introduced in the Information Management System (IMS) developed by IBM in 1968. This model is like a hierarchical tree structure, used to construct a hierarchy of records in the form of nodes and branches. The data elements present in the structure have Parent-Child relationship. Closely related information in the parent-child structure is stored together as a logical unit. A parent unit may have many child units, but a child is restricted to have only one parent.
The drawbacks of this model are :
● The hierarchical structure is not flexible to represent all the relationship proportions, which occur in the real world.
● It cannot demonstrate the overall data model for the enterprise because of the non-availability of actual data at the time of designing the data model.
● It cannot represent the Many-to-Many relationship.
Network Model:
It supports the One-To-One and One-To-Many types only. The basic objects in this model are Data Items, Data Aggregates, Records and Sets.
It is an improvement on the Hierarchical Model. Here multiple parent-child relationships are used. Rapid and easy access to data is possible in this model due to multiple access paths to the data elements.
Relational Model:
● Does not maintain physical connection between relations
● Data is organized in terms of rows and columns in a table
● The position of a row and/or column in a table is of no importance
● The intersection of a row and column must give a single value
● Features of an RDBMS o The ability to create multiple relations and enter data into them o An attractive query language o Retrieval of information stored in more than one table
● An RDBMS product has to satisfy at least Seven of the 12 rules of Codd to be accepted as a full- fledged RDBMS.
● Hierarchical
● Network
● Relational
Hierarchical Model:
This model was introduced in the Information Management System (IMS) developed by IBM in 1968. This model is like a hierarchical tree structure, used to construct a hierarchy of records in the form of nodes and branches. The data elements present in the structure have Parent-Child relationship. Closely related information in the parent-child structure is stored together as a logical unit. A parent unit may have many child units, but a child is restricted to have only one parent.
The drawbacks of this model are :
● The hierarchical structure is not flexible to represent all the relationship proportions, which occur in the real world.
● It cannot demonstrate the overall data model for the enterprise because of the non-availability of actual data at the time of designing the data model.
● It cannot represent the Many-to-Many relationship.
Network Model:
It supports the One-To-One and One-To-Many types only. The basic objects in this model are Data Items, Data Aggregates, Records and Sets.
It is an improvement on the Hierarchical Model. Here multiple parent-child relationships are used. Rapid and easy access to data is possible in this model due to multiple access paths to the data elements.
Relational Model:
● Does not maintain physical connection between relations
● Data is organized in terms of rows and columns in a table
● The position of a row and/or column in a table is of no importance
● The intersection of a row and column must give a single value
● Features of an RDBMS o The ability to create multiple relations and enter data into them o An attractive query language o Retrieval of information stored in more than one table
● An RDBMS product has to satisfy at least Seven of the 12 rules of Codd to be accepted as a full- fledged RDBMS.
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