Object oriented databases are also called Object Database Management Systems (ODBMS). Object databases store objects rather than data such as integers, strings or real numbers. Objects are used in object oriented languages such as Smalltalk, C++, Java, and others. Objects basically consist of the following:
- Attributes – Attributes are data which defines the characteristics of an object. This data may be simple such as integers, strings, and real numbers or it may be a reference to a complex object.
- Methods – Methods define the behavior of an object and are what was formally called procedures or functions.
Therefore objects contain both executable code and data. There are other characteristics of objects such as whether methods or data can be accessed from outside the object. We don’t consider this here, to keep the definition simple and to apply it to what an object database is. One other term worth mentioning is classes. Classes are used in object oriented programming to define the data and methods the object will contain. The class is like a template to the object. The class does not itself contain data or methods but defines the data and methods contained in the object. The class is used to create (instantiate) the object. Classes may be used in object databases to recreate parts of the object that may not actually be stored in the database. Methods may not be stored in the database and may be recreated by using a class.
Comparison to Relational Databases
Relational databases store data in tables that are two dimensional. The tables have rows and columns. Relational database tables are “normalized” so data is not repeated more often than necessary. All table columns depend on a primary key (a unique value in the column) to identify the column. Once the specific column is identified, data from one or more rows associated with that column may be obtained or changed.
To put objects into relational databases, they must be described in terms of simple string, integer, or real number data. For instance in the case of an airplane. The wing may be placed in one table with rows and columns describing its dimensions and characteristics. The fusalage may be in another table, the propeller in another table, tires, and so on.
Breaking complex information out into simple data takes time and is labor intensive. Code must be written to accomplish this task.