Learn software concepts - 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

Entity - Relationship Model

The entity-relationship (ER) data model allows us to describe the data involved in a real-world enterprise in terms of objects and their relationships and is widely used to develop an initial database design. The ER model is important primarily for its role in database design. It provides useful concepts that allow us to move from an informal description of what users want from their database to a more detailed, and precise, description that can be implemented in a DBMS.

ENTITIES, ATTRIBUTES, AND ENTITY SETS
- A database can be modeled as:
o a collection of entities,
o relationship among entities.
- An entity is an object that exists and is distinguishable from other objects.
o Example: specific person, company, event, plant
- Entities have attributes.
o Example: people have names and addresses
- An entity set is a set of entities of the same type that share the same properties.
o Example: set of all persons, companies, trees, holidays.



- Attributes : An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set.
o E.g. Employee = (Name, Address, Age, Salary)



Types of Attributes
- SIMPLE attributes are attributes that are drawn from the atomic value domains.
E.g. Name = {John} ; Age = {23}
- COMPOSITE attributes: Attributes that consist of a hierarchy of attributes.
E.g. Address may consists of “Number”, “Street” and “Suburb” → Address = {59 + ‘Meek Street’ + ‘Kingsford’}
- SINGLE VALUED attributes: Attributes that have only one value for each entity.
E.g. Name, Age for EMPLOYEE
- MULTIVALUED attributes: Attributes that have a set of values for each entity.
E.g. Degrees of a person: ‘ BSc’ , ‘MIT’, ‘PhD’
- DERIVED attributes: Attributes Contain values that are calculated from other attributes.
E.g. Age can be derived from attribute DateOfBirth. In this situation, DateOfBirth might be called Stored Attribute.

- DOMAIN/VALUE SETS : Each simple attribute of an entity type is associated with a value set which specifies the set of values that may be assigned to that attribute for each individual entity.
- KEYS : An important constraint on the entities is the key. An entity type usually has an attribute whose values are distinct for each individual entity in the collection. Such an attribute is called a key attribute.
For example, for the entity SET EMPLOYEE = {EID, Name, Address, Age, Salary}
EID attribute is the unique key for entity set EMPLOYEE as no two employees can have two same EID. Some entity types have more than one key attribute. An entity type may also have no key.

Entity - Relationship Model

The entity-relationship (ER) data model allows us to describe the data involved in a real-world enterprise in terms of objects and their relationships and is widely used to develop an initial database design. The ER model is important primarily for its role in database design. It provides useful concepts that allow us to move from an informal description of what users want from their database to a more detailed, and precise, description that can be implemented in a DBMS.

ENTITIES, ATTRIBUTES, AND ENTITY SETS
- A database can be modeled as:
o a collection of entities,
o relationship among entities.
- An entity is an object that exists and is distinguishable from other objects.
o Example: specific person, company, event, plant
- Entities have attributes.
o Example: people have names and addresses
- An entity set is a set of entities of the same type that share the same properties.
o Example: set of all persons, companies, trees, holidays.



- Attributes : An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set.
o E.g. Employee = (Name, Address, Age, Salary)



Types of Attributes
- SIMPLE attributes are attributes that are drawn from the atomic value domains.
E.g. Name = {John} ; Age = {23}
- COMPOSITE attributes: Attributes that consist of a hierarchy of attributes.
E.g. Address may consists of “Number”, “Street” and “Suburb” → Address = {59 + ‘Meek Street’ + ‘Kingsford’}
- SINGLE VALUED attributes: Attributes that have only one value for each entity.
E.g. Name, Age for EMPLOYEE
- MULTIVALUED attributes: Attributes that have a set of values for each entity.
E.g. Degrees of a person: ‘ BSc’ , ‘MIT’, ‘PhD’
- DERIVED attributes: Attributes Contain values that are calculated from other attributes.
E.g. Age can be derived from attribute DateOfBirth. In this situation, DateOfBirth might be called Stored Attribute.

- DOMAIN/VALUE SETS : Each simple attribute of an entity type is associated with a value set which specifies the set of values that may be assigned to that attribute for each individual entity.
- KEYS : An important constraint on the entities is the key. An entity type usually has an attribute whose values are distinct for each individual entity in the collection. Such an attribute is called a key attribute.
For example, for the entity SET EMPLOYEE = {EID, Name, Address, Age, Salary}
EID attribute is the unique key for entity set EMPLOYEE as no two employees can have two same EID. Some entity types have more than one key attribute. An entity type may also have no key.