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141 lines
4.0 KiB
Markdown
141 lines
4.0 KiB
Markdown
---
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title: "13-databases-1"
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aliases:
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tags:
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- cosc203
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- lecture
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sr-due: 2022-09-25
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sr-interval: 3
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sr-ease: 250
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---
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# What is a database
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- an integrated collection of data
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- implicit properties
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- represents some aspect of th real world, called miniworld
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- is a coherent collection of data with inherent meaning
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- is designed built and populated with data for a specific purpose
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## DBMS
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- a software system designed to store, manage, and facilitate access to the database
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- Oracle, SQL erver, MySQL, MonogDB
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## Application program
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- a program that provides access to the dtabase by sending queries and requests for data to the DBMS
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# This course
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- modelling and design of DBs
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- programming (SQL + appliation development)
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- DBMS implementation
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specifiically *relational databases*
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- ER modelling
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- datbase normalisation
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- SQL
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## relational vs nosql
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relation is better for structured data, nosql is better for abstract data, as it is more flexible
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# Data modelling
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first step in designing database
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- create a data model for a specific problem domain
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a model is Defn: a collection of concepts that can be used to describe the structure of a database such as data types, relationsips, and contraints that should hold for the data
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schema Defn: a collection of metadata that describes the logical view of a database
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noSQL databases are either schema-free or have relaxed schemas
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## conceptual data modelling
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model the data close to the way the users percieve the data
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ER model
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- popular high level conceptual model
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- uses diagrammatic notations - er diagram (ERD)
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three notations
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- chens notation
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- crows foot notation
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- UML notation
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# Example
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# how to model
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## entities
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- a thing in the real world with independent existence
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- physical existence
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- conceptial existence
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entity type is a collection of entities with the same attributes — represented by a rectangle
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## attribute
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- a property of an entity
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- simple (atomic) e.g, first name
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- composite (can be divided) e.g., address
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- single value e.g., id number
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- multi value e.g., siblings
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- stored - by default
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- derived - drawn as dashed ovals e.g., age is derived from birth date
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- key
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- unique
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- simple or composite
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- underlined in ERD
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## relationships
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a relationship model the connection between entitiy types
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e.g., each employee works for one department
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relationship type is a set of relationsihps among entities from the particular entity types
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- represented by diamonds
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relationship types can have attributes
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properties
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- degree
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- binary [binary relationship|200](https://i.imgur.com/QDpdMo7.png)
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- n-ary [n-ary relationship|300](https://i.imgur.com/7oIJP5j.png)
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- cardinality for binary relationships
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- one to one
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- one to n
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- m to n
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- participation
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- total participation
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- marked by double line
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- relationship is required
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- partial
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- marked by single line
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- relationsihp is not required
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## weak entities
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- regular types have key attributes
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- weak types do not have their own key attributes
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- they are identified by their relationship with another entity
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- this creates and identifying relationship
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- must have a total participation constraint
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- the other entity type is called the identifying or owner entity type
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- denoted by double lined rectable and double lined diamond
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- have a partial key
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- a set of attributes that uniquiely identify the weak entities that are related to the same owner entity
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