2.2. Theory UML ERD

• UML - Unified Modeling Language

• ERD - Entity Relationship Diagram

2.2.1. Entities vs. Attributes

• Entities - rows in database

• Attributes - columns in database

2.2.2. Cardinality

Cardinality:

Table 2.5. Cardinality

left	right	description
|o	o|	zero or one
||	||	exactly one
}o	o{	zero or more (no upper limit)
}|	|{	one or more (no upper limit)

Relationship:

Table 2.6. Cardinality

cardinality	relationship
||..||	one to one
||..|{	one to many
}|..||	many to one
}|..|{	many to many

2.2.3. Entity Relationship Diagram

• ERD - Entity Relationship Diagram [4]

• By drawing a line between tables we show, that there is a connection between them in some way

• English: "crows foot notation"

• Polish: "notacja kurzych stóp"

Right:

• ASSIGNMENT relates to one MISSION

• ASSIGNMENT assigns one ASTRONAUT

• ASSIGNMENT defines one ROLE

Left:

• MISSION is related to zero or many ASSIGNMENT

• ASTRONAUT is assigned to zero or many ASSIGNMENT

• ROLE is defined in one ASSIGNMENT

```mermaid
erDiagram

ASSIGNMENT }o..|| MISSION : "relates to"
ASSIGNMENT }o..|| ASTRONAUT : "assigns"
ASSIGNMENT ||..|| ROLE : "defines"

ASSIGNMENT {
    primary_key id
    foreign_key mission_id
    foreign_key astronaut_id
    str role
}

ROLE {
    primary_key id
    str name
}

ASTRONAUT {
    primary_key id
    str firstName
    str lastName
    int age
}

MISSION {
    primary_key id
    int year
    int name
}
```

2.2.4. Mermaid

• mermaid - Markdown extension [1]

Theming [2]:

%%{init: { 'theme': 'dark' } }%%
%%{init: { 'theme': 'forest' } }%%

Config [3]:

%%{init: { 'logLevel': 'debug' } }%%
%%{config: { 'fontFamily': 'Menlo', 'fontSize': 18, 'fontWeight': 400} }%%

Table 2.7. CSS classes

Selector	Description
.er.attributeBoxEven	The box containing attributes on even-numbered rows
.er.attributeBoxOdd	The box containing attributes on odd-numbered rows
.er.entityBox	The box representing an entity
.er.entityLabel	The label for an entity
.er.relationshipLabel	The label for a relationship
.er.relationshipLabelBox	The box surrounding a relationship label
.er.relationshipLine	The line representing a relationship between entities

2.2.5. Use Case - 1

```mermaid
erDiagram

CUSTOMER }|..|{ DELIVERY-ADDRESS : has
CUSTOMER ||--o{ ORDER : places
CUSTOMER ||--o{ INVOICE : "liable for"
DELIVERY-ADDRESS ||--o{ ORDER : receives
INVOICE ||--|{ ORDER : covers
ORDER ||--|{ ORDER-ITEM : includes
PRODUCT-CATEGORY ||--|{ PRODUCT : contains
PRODUCT ||--o{ ORDER-ITEM : "ordered in"
```

2.2.6. Use Case - 2

2.2.7. References

[1]

Sveidqvist, Knut et al. Mermaid Documentation: About. Year: 2022. Retrieved: 2022-01-26. URL: https://mermaid-js.github.io/mermaid/#/README

[2]

Sveidqvist, Knut et al. Mermaid Documentation: Theming. Year: 2022. Retrieved: 2022-01-26. URL: https://mermaid-js.github.io/mermaid/#/theming

[3]

Sveidqvist, Knut et al. Mermaid Documentation: SyntaxReference. Year: 2022. Retrieved: 2022-01-26. URL: https://mermaid-js.github.io/mermaid/#/n00b-syntaxReference

[4]

Sveidqvist, Knut et al. Mermaid Documentation: Entity Relationship Diagrams. Year: 2022. Retrieved: 2022-01-26. URL: https://mermaid-js.github.io/mermaid/#/entityRelationshipDiagram