Week 1 ER Modelling - Notes PDF

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Week 1 : ER Modelling Tuesday, 4 June 2019 2:27 PM

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Data Modelling

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A model is an abstraction of a real-world Data modelling is simple representation of complex data structure Data modelling can be classified based on the degree of abstraction Conceptual modelling Internal modelling External modelling Physical modelling

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Conceputal Data Modelling A concepyual data model is a representation of organizational data. 2 techniques:

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Entity- Relationship (ER) modelling: Top-down [自顶向下]approach. Begains by looking for the data groups in the system Normalization: Bottom-up approach. Begains by looking at the samllest individual items of data recorded by the system

Internal Model Used when database is implemented Maps the conceptual model to the DBMS [将概念模型映射到 DBMS] Internal schema depicts a specific representation of a internal model Depends on the specific database software → a change in DBSM software requies internal model be changed. Logical independence: you can change the internal model without affecting conceptual model

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Conceptual Model vs. Internal Model

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Conceptual Modelling: ER Model ER model: A detailed, logical representation of data for an organization or for a business area. Expressed in terms of entitites in the business enviroment, the relatioships or associations among those entities, and the attributes of both the entities and their relatioship ER model is normally expressed as an ER diagram, which is a grograohical representaion of an ER model [Chen Model]

2.Entity (Type) and Entity (Instance)  o o 

Entity: A thing with distinct and independent existence [一种独立存在的东西] Existenc, being Synonyms: being - existence - essence - thing Instance: A example or single occurance of something: "An instance of corruption"

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A particular case: "in this instance." Synonyms: example - case - sample - event - occurrence - exemplar Example： ：One type of things is a Drink. Bubble tea is an instance of Drink Entities: Is an object about which the system requires to hold data Entity type: A collection of entities, that share commen properties or characteristics Entitiy instance: A single occurrence of an entity type

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3.Attributes  

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Attributes: A property or characteristics of an entity that is interest to the organization Each entity type has a set of general attributes associated with it. o E.g. The entity typr STUDENT has the attributes "Student id", "StudentName" , "ProgramName" Each entity instance has specific values of the attributes associate with it o The entity instance S. LAW has the attribute values “S221”; “Law, S.” and “SE”. Attributrs have domains. Domains: attribute's set of possible values A simple attributes cannot be subdivided o E.g. ZID A composite attributes is a super-set of sub-attributes A single-valued attributes can only have one value (simple or composite) A multi - valued attributes can have many values A derived attibutes is derived using an algorithm [通过计算获得的] A key attributes is unique so to identify the entity

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4. Keys 

Key: An attributes or set of attributes whose values uniquely identify one occurrence of thay entity o Candidate Key: It is an attribute that uniquely identifies each instance of an entity type o Primary Key: Is is a candidate key that has been selected to be used as an identifier for an entity type o Foreign key : An entity may contain as one of its attributes a data item [ 作为其属性之一的数据项 ]which is the primary key of another entity

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Relationship: Is a link between 2 entities which is significant for the system Degree of a relationship is the number of entities types that participate in that relationship o Unary, binary, ternary, quaternary

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Connnectivity Connectivity: Describe the relationship classification

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7.Cardinality   

Cardinality: Express the soecific number of entity occurences associated with one occurrence of related entity Cardinality constraint: Specifies the number of instances of entity A that can be associated with each insatnce of entity B. Business rules: They are derived from organization's data enviroment

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Minimum caridinality: the minimum number of instances of one entity that may be associated with each instance of another entity Maximum cardinality: the maximum number of insyance that might be associated with each instance of another entity.

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8.weak Entities 

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Weak entities: Entity that relies on the existance of another (strong ot independt) entity.It has a primary key that is partially or totally derived from the parent entity in the relationship Weak entity meets 2 conditions： o Existence -dependent: Cannot exist without entity which it has a relationship o Has a primary key that is partially or totally derived from the parent entity in the relatioship Database designer usually determined whether an entity can be described as weak based on the business rule...