Data Modelling

1, data modelling is a technique for organizing and documenting a systems data in a model.

2. ERD depicts data in term of the entities and relationships described by the data.

3, Relational Data Model is a blueprint for implementation of a conceptual data model ( ERD) in relational database enviroment( software indepenndent)

4, MS Acess Relationship Window: a graph showing how a data model is implemented with Microsoft Acces

ERD

- is used to model data and their relationship

- is a graphical representation of a conceptual data model

- is resource independent: it does not comit to any particular database environment.

ENTITY

- is a group of attributes corresponding to the same conceptual object about which we need to capture and store data.

- is a set of occurrences(instances) of the object that it represents

- must have a unique name ( a singular noun) unique identifier, and at least one attribute ( the identifier itself)

ATTRIBUTES

- is a descritptive property or characteristic of interest of an entiy. Also called element, property and field

- DATA TYPE for an attribute defines what type of data can be sorted in that attribute

- DOMAIN of an attribute defines what values an attribute can legitimately take on.

- DEFAULT VALUE for an attributes is the value that will be recorded if not specified by the user.

IDENTIFICATION

- a KEY is an attribute, or a group of attributes that assume a UNIQUE VALUE for each entity instance

- a group of attributes that uniquely identifies an instance of a entity is called a COMPOSITE KEY

- a PRIMARY KEY ( identifier) is that candidate key that will most commonly be used to uniquely identify a single entity instance( StudentID)

- Any candidate key that's not selected to become the primary key is called an alternative key. ( SSN, DriverLicenseNo)

- a SECONDARY KEY is an attribute whose values divide all entity instances into usefull subgroup/ sub-criteria ( Major, Gender..)

RELATIONSHIPS

- a relationship documents an asscociation between one, two, or more entites

- must have a name( and many carry data)

- Degree of relationshiip ( number of entities)

- Cardinalities of relationship ( number of instances / members)

DEGREE OF RELATIONSHIp defines how many entiies are involved in a relationship.

exmalple: Recursive ( unary), Binary, ternary

CARDINALITIES

- document how many occurrences/member of one entity can relate to a single occurrence/ member of another entity in a relationship

- Max / MIx number of occurrences

- Reflect business policies or general business practices ( how many classes a student can take, how many students a class can hold)

LOGICAL DATA MODELLING STAGES

1. Context Data Model

- To establish project scope

2. Key-base data model

- Eliminate nonspecific relationships

- Add associative entities

- Include primary and alternate keys

- Precise cardinalities

3. Fully attributed data model

- All remaining attributes

- Subsetting criteria

4. Normalized data model

GOOD DATA MODEL

- Simple

Data attributes that describe any entity should describe only that entity.

Each attribute of an entity instances can have only one value.

- Non redundant:

Each data attribute, other than foreign keys, describes at most one entity

Look for the same attributes recorded more than once under different names.

- Flexible and adaptable

Rules In ERD BUIDING

- Each entitly must have a name

- Each entity must have an identifier

- An occurrence itself cannot be an entity

- Each relationship must have a name

- Reasonable cardinalities ( context specific)

ASSOCIATIVE ENTITY

- is an entity that inherits its primary key from more than one other entity ( called parents)

- each part of composite key points to one and only one instance of each of the connecting entities

FOREIGN KEYS IN RELATIONAL DATABASE

- a foreign key in entity is a primary key of another entity, which is used to identify a one-to-many relationship between two other entities. \

- In M:N relationship, the associative /junction table with a composite key will be used to capture the relationship.

- Each part of the composite key serves like a foreign key

- sometimes, a surrogate key ( RecordNo) is used as primary key to simplify the identification of record.

DATA ANYALYSIS NORMALIZATION

- data analysis is a process that prepares a data model for implementation as a simple, non-redundant, flexible, and adaptable database.

- is technique to organize data attributes such that they are grouped to form non redundant, stable, flexible, and adaptive entities.

1NF:

- No repeating group of same attribute ( multi valued attributed)

2NF

- Attributes should depen on the whole ( composite) key, not part of it( partial functional dependency)

3NF:

- Attributes should depend on the primary key only, not on each other.( transitive dependency)