preface_3.htm
A database is supposed to store data. A data object is a storage space holding values or information which can be identified by name-type or data identifier.

Data type: The data type like  integer, double, float, char , are fundamental data types. The class, struct , pointers, and enum are derived data types. The scalar data, like pointer and arithmatic data type, contains one single data value where as aggregate types, like arrays, struct,  would have multiple values.

SQL and various programmatic interfaces and languages, including PL/SQL, Java, Oracle Call Interface, Pro*C/C++, OO4O, and C# have been enhanced with extensions to support Oracle objects. The result is an object-relational model that offers the intuitiveness and economy of an object interface while preserving the high concurrency and throughput of a relational database.

In oracle
COMPOSITE DATA TYPES ARE---

1.RECORD TYPE
2.NESTED TABLE
3.VARRYING ARRAY
Aggregate data types

• class, arrays, string and struct

 The struct comes from C, and  class from C++. The instance of a class is called an object.

The struct and class both are aggregate types, meaning that they group together multiple fields of data
• for example: struct point { double x, y; };
 
UML distinguishes two kind of aggregation, simple aggregation and composed aggregation.

 
Simple Aggregation: is a logical aggregation where elimination of one participants will not eliminate the this logical association or other participants. The simple aggregation does not imply any restriction over the other part. Here teacher to departments a simple aggregation,  teacher block ( e.g. online courses) can be dropped and recreated without affecting the departments.
Composite or Composed-Aggregation:  is a special kind of aggregation in which the parts are physically bonded as a whole unit, and a part of this aggregation,  lives and dies with its whole. Here the existence of student block is must to the teacher block. The elimination of student block, also eliminate teacher block. 

Composition implies a common ownership, where as an aggregation is a mere relationship.
   
  Relations Aggregation Composition
Owner No owner Single owner Single owner
Life time Each has independent  lifetime Each has it's own  lifetime Owner's life time, enforces the life of the member objects.
Child object The objects all are independent All the child objects belong to a single parent All the child objects belong to a single parent.