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Comparison Between Relational Database Model and Object Oriented Model

Now we know about both relational and object oriented approach, we can now compare these two models. In this session, we compare the relational model and object oriented model. We compare model representation capabilities, languages, system storage structures, and integrity constraints.

Data Model Representation

Different database models differ in their representation of relationships. In relational model, connections between two relations are represented by foreign key attribute in one relation that reference the primary key of another relation. Individual tuples having same values in foreign and primary key attribute are logically related. They are physically not connected. Relational model uses logical references.

In object oriented model, relationships are represented by references via the object identifier (OID). This is in a way similar to foreign keys but internal system identifiers are used rather than user-defined attributes. The Object Oriented model supports complex object structures by using tuple, set, list, and other constructors. It supports the specification of methods and the inheritance mechanism that permits creation of new class definitions from existing ones.

Storage Structures

In relational model, each base relation is implemented as a separate file. If the does not specify any storage structure, most RDBMS will store the tuples as unordered records in the file. It allows the user to specify dynamically on each file a single primary or clustering index and any number of secondary indexes. It is the responsibility of user to chObject Orientedse the attributes on which the indexes are set up. Some RDBMSs give the user the option of mixing records from several base relations together. It is useful when related records from more than one relation are often accessed together. This clustering of records physically places a record from one relation followed by the related records from another relation. In this way the related records may be retrieved in most efficient way possible.

Object Oriented systems provide persistent storage for complex-structured objects. They employ indexing techniques to locate disk pages that store the object. The objects are often stored as byte strings, and the object structure is reconstructed after copying the disk pages that contain the object into system buffers.

Integrity Constraints

Relational model has keys, entity integrity, and referential integrity. The constraints supported by Object Oriented systems vary from system to system. The inverse relationship mechanism supported by some Object Oriented systems provides some declarative constraints.

Data manipulation Languages

There are languages such as SQL, QUEL, and QBE are available for relational systems. These are based on the relational calculus.

In Object Oriented systems, the DML is typically incorporated into some programming language, such as C++. Hence, the structures of both stored persistent objects and programminglanguage transient objects are often compatible. Query languages have been developed for Object Oriented databases. Work on a standard Object Oriented model and language is progressing, but no complete detailed standard has emerged as yet.

See Also


  

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