What are the different data modeling concepts?

Data model is a conceptual representation of data structures. The data structures consist of data objects, relationship between data objects and rules that govern these operations. Often, analysis modeling begins with data modeling.
The inputs of data model comes from the planning and analysis stage. There are two outputs of the data model. First is an entity relationship diagram and second is a data document. The goal of the data model is to make sure that the all data objects required by the database are completely and accurately represented.

The different concepts of data modeling are:

- Data Objects are a representation of any composite information that is processed by software. A data object can be an external entity, thing, occurrence, event, role, an organizational unit, place or a structure. The description of the data object includes data object and its attributes. A data object contains only data.

- Data Attributes name a data object, describe its characteristics and sometimes make reference to another object. One or more attributes must be identified as a key which acts as an identifier.

- Relationships indicate the manner in which data objects are connected to one another.

- Cardinality of a relationship is the actual number of related occurences for each of the two entities. It defines the maximum number of objects participating in relationship. It does not indicate whether a data object should participate in relationship or not.

- Modality of a relationship can be 0 or 1. It is 1 if an occurrence of relationship is must. It is 0 if an occurrence of relationship is optional.

Object Definition language (ODL)

Object Definition Language (ODL) is the specification language defining the interface to object types conforming to the ODMG Object Model. Often abbreviated by the acronym ODL.
This language's purpose is to define the structure of an Entity-relationship diagram.

- Class Declarations
* interface < name > {elements = attributes, relationships,methods }
- Element Declarations
* attribute < type > < name >;
* relationship < rangetype >< name >;
- Method Example
* float gpa(in: Student) raises(noGrades)
# float = return type.
# in: indicates Student argument is read-only.
Other options: out, inout.
# noGrades is an exception that can be raised by method gpa.

ODL Type System :
- Basic types: int, real/ float, string, enumerated types, and classes.
- Type constructors: Struct for structures and four collection types: Set, Bag, List, and
Array.

ER versus ODL :
- E/R: arrow pointing to "one".
- ODL: don't use a collection type for relationship in the “many" class.
* Collection type remains in “one.”
- E/R: arrows in both directions.
- ODL: omit collection types in both directions.
- ODL only supports binary relationship.
- Convert multi-way relationships to binary and then represent in ODL.
- Create a new connecting entity set to represent the rows in the relationship set.
- Problems handling cardinality constraints properly!!

Keys in ODL :
- Indicate with key(s) following the class name, and a list of attributes forming the key.
* Several lists may be used to indicate several alternative keys.
* Parentheses group members of a key, and also group key to the declared keys.
* Thus, (key(a1; a2; : : : ; an )) = “one key consisting of all n attributes." (key a1; a2; : : : ; an ) =“each ai is a key by itself.
- Keys are not necessary for ODL. Object identity and not keys differentiates objects