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Framed out notes. Notes 01 and 02
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‎OMSCS/Courses/DB/01 - Fundamentals of Databases.md‎

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---
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tags:
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- OMSCS
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- DB
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---
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# 01 - Fundamentals of Databases
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- high level overview of what a DB is
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- it's a "model of reality"
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- why use models at all?
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- when to use a Database Management System (DBMS)
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## Models of Reality
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- a model is a means of communication
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- users of a model must have a certain amount of knowledge in common
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- a model
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- only emphasizes selected aspects
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- is described in some language
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- can be erroneous
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- may have features that do not exist in reality
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## To use or not to use a DBMS
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### To Use
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- data-intensive applications
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- persistent storage of data
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- centralized control of data
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- control of redundancy
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- control of consistency and integrity
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- consistency = whether you can derive contradictions from within the DB itself
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- multiple user support
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- flight reservation
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- point of sale transactions
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- sharing of data
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- data documentation
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- data independence
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- control of access and security
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- backup and recovery
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### Not To Use
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- the initial investment in hardware, software, and training is too high
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- the generality is not needed
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- overhead for security, concurrency, and recovery is too high
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- data and apps are simple and stable
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- real-time requirements cannot be met by it
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- multiple user access is not needed
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## Outline of Major Topics
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- data modeling
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- process modeling
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- database efficiency
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## Data Modeling
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![[Pasted image 20250831114949.png]]
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- the model represents a perception of structures of reality
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- the data modeling process is to fix a perception of structures of reality and represent this perception
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- in the data modeling process, we select aspects and abstract
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## Process Modeling
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![[Pasted image 20250831114933.png]]
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- the use of the model reflects processes of reality
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- processes may be represented
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- embedded in program code
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- executed ad-hoc
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![[Pasted image 20250831114914.png]]
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## Data Models
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- data structures
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- constraints
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- operations
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- keys / identifiers
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- integrity / consistency
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- null values
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- surrogates
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## Architecture
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- database
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- ANSI/SPARC 3-Level DB Architecture
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- data independence
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- DBMS
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## Metadata
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## Example of Data Models
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> A data model is not the same as a model of data.
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- Entity-Relationship Model
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- Relational Model
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- Hierarchical Model (legacy, IBM IMS, XML)
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### Relational Model
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#### Data Structures
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- data is represented in tables
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- tables have a name
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- tables have columns
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- columns have a data type
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- tables have rows
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- schema represents aspects of the data (the structure)
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- The schema is not expected to change (much)
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#### Constraints
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- constraints express rules that cannot be expressed by the data structures alone (more than just type constraints)
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- validation rules
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- foreign key relations
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- unique constraints
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- > dates must be after "1900-01-01"
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#### Operations
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- operations support change and retrieval of data
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- CRUD operations
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- list operation
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- filtering
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- etc
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## Keys and Identifiers
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- keys are uniqueness constraints
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- keys are used for reference and lookup of rows
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## Integrity and Consistency
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- **integrity**: Does the DB reflect reality well?
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- **consistency**: Is the DB without internal conflicts?
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## Null Values
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- it's "advanced 0"
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- represents the lack of a value, not a value itself
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- also represents values which are "inapplicable" to the specific row ("catch-all" forms)
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## Surrogates - Things and Names
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- "Leo"
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- "GTO1"
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- "49"
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- **name-based**: a thing is what we know about it
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- surrogates are system-generated, unique, internal identifiers
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![[Pasted image 20250831120057.png]]
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![[Pasted image 20250831120320.png]]
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## ANSI/SPARC 3-Level DB Architecture
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### Separating Concerns
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- a DB is divided into schema and data
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- the schema describes the intention (types)
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- the data describes the extension (data)
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![[Pasted image 20250901150105.png]]
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- benefits include
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- it's possible to change how data is stored without changing the application which uses the data
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- physical data independence is a measure of how much the internal schema can change without affecting the application programs
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- logical data independence is a measure of how much the conceptual schema can change without affecting the application programs
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### Conceptual Schema
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- describes conceptually relevant, general, time-invariant structural aspects of reality
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- excludes aspects of data representation, physical organization, and access
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- applications can only "see" these structures
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### External Schema
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- describes parts of the information in the conceptual schema in a form convenient to a particular user group's view
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- is derived from the conceptual schema
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### Internal Schema
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- describes how the information described in the conceptual schema is physically represented to provide the overall best performance
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- includes indexes
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## ANSI/SPARC DBMS Framework
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![[Pasted image 20250901150809.png]]
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- hexagons are different people/roles
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- triangle is where schema definitions are stored
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- squares are processors
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- 2 main parts
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- schema compiler
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- query transformer
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## Metadata
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- system metadata
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- where data came from
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- how data is changed
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- how data is stored
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- how data is mapped
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- who owns data
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- who can access data
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- data usage history
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- data usage statistics
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- business metadata
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- what data is available
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- where data is located
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- what the data means
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- how to access data
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- predefined reports
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- predefined queries
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- how current the data is
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- importance
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- system metadata is critical in a DBMS
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- business metadata is critical in a data warehouse

‎OMSCS/Courses/DB/02 - Extended Entity-Relationship (EER) Model.md‎

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---
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# 02 - Extended Entity-Relationship (EER) Model
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> In order to do data modeling, we need data models.
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## Entity Type and Entity Surrogates
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![[Pasted image 20250901153806.png]]
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- entity types are represented as rectangles
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- entity type names must be unique
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## Single Valued Properties
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- sv props are represented as ellipses
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- these ellipses are linked to an entity type
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- prop values are
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- lexical, visible, audible
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- they are things which name other things
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- properties identify another property if they are "underlined"
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- for each identifying prop value, there is at most one instance of the identified entity
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- each entity must be uniquely referenceable
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## Composite Properties
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- composite props are represented as ellipses
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- these ellipses are linked to another property
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- the "name" property is a composite property, composed of firstname and lastname
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![[Pasted image 20250901154125.png]]
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## Multi-Valued Property
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- double ellipses
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![[Pasted image 20250901154158.png]]
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## Relationships
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- relationships are represented as diamonds
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- the names of multiple relationship types between the same two entity types must be unique
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### 1-1 relationship types
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![[Pasted image 20250901154343.png]]
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- partial function
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### 1-many relationship types
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![[Pasted image 20250901154509.png]]
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- partial function
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### mandatory 1-N relationship types
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![[Pasted image 20250901154637.png]]
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- total function
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### N-M relationship types
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![[Pasted image 20250901154728.png]]
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### N-ary relationship types
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![[Pasted image 20250901154836.png]]
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- relationships can link 2 or more entities
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### Many relationship types
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> Many ternary relationship types cannot be reduced to a conjunction of binary relationship types.
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![[Pasted image 20250901154955.png]]
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### Identifying relationships / weak entity types
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![[Pasted image 20250909204641.png]]
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- StatusUpdate is identified by the email of the user and the date/time it was posted.
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- Cannot exist without RegularUser
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- Cannot be identified without RegularUser (ID: (Email, DateAndTime) tuple)
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- StatusUpdate is a weak identity because its ID has to go "through" the RegularUser entity
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### Recursive Relationship Types
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![[Pasted image 20250909204929.png]]
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- Creates a graph or tree structure within a single entity type (or a set of entity types)
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## Supertypes and subtypes
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### "is-a" relationship types
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![[Pasted image 20250909205033.png]]
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- "d" is "disjoint"
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- "o" is "overlap"
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### Inheritance
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![[Pasted image 20250909205209.png]]
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## Union Entity Types
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![[Pasted image 20250909205410.png]]
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- example above, employer can be Company or GovtAgency
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- $Employer \subseteq Company \cup GovtAgency$
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- $Compay \cap GovtAgency = \emptyset$
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## Are Relationships Entities?
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> Or are they just glue?
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![[Pasted image 20250909205920.png]]
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- relationships may have attributes
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- for 1-N relationships, attributes may be moved to the entity on the "many-side".
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- for 1-1 relationships, attributes may exist on either entity
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![[Pasted image 20250909210121.png]]
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- The example above is an "objectified relationship type"
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## Fun Example
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![[Pasted image 20250909210451.png]]
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## What can the EER do?
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- Classification
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- Generalization
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- Does not do aggregation
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- You can't model a drive train in an EER model
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## What is the result type of a query?
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![[Pasted image 20250909210853.png]]
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- list of properties are not entity types
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- there is no "type"
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- DBMSes are not based on EER
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## Relational Model
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- Data structures
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- Constraints
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- Operations
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- Relational Algebra
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- Relational Calculus
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- Tuple Calculus (SQL)
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- Domain Calculus (QBE)
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### Data Structures
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- There is only one structure (relations)
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- a domain $D$ is a set of atomic values
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- a relation $R$ is a subset of the set of ordered n-tuples
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- ![[Pasted image 20250909211306.png]]
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- an attribute $A$ is a unique name given to a domain in a relation helping us interpret domain values
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> We illustrate domains by tables.
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![[Pasted image 20250909211330.png]]
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> The value of a relation is independent of attribute order and tuple order.
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### Constraints
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- Keys
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- Primary Keys
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- Entity integrity
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- Referential integrity
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‎OMSCS/Courses/DB/03 - EER Mapping.md‎

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# 02 - EER Mapping

‎OMSCS/Courses/DB/04 - Relational Algebra and Calculus.md‎

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# 04 - Relational Algebra and Calculus

‎OMSCS/Courses/DB/05 - SQL.md‎

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# 05 - SQL

‎OMSCS/Courses/DB/06 - Normalization.md‎

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# 06 - Normalization

‎OMSCS/Courses/DB/07 - Efficiency and Indexing.md‎

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# 07 - Efficiency and Indexing

‎OMSCS/Courses/DB/08 - Metadata (bonus lecture).md‎

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# 08 - Metadata (bonus lecture)

‎OMSCS/Courses/DB/Exam 1.md‎

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# Exam 1
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- Textbook chapters: 1, 2, 3, 4
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- [[01 - Fundamentals of Databases]]
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- [[02 - Extended Entity-Relationship (EER) Model]]

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