Entity Relationship Diagrams

《An entity-relationship model (or ER model) describes interrelated things of interest in a specific domain of knowledge. A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between entities (instances of those entity types)》 (Wikipedia)

Note that practitioners of ER modelling almost always refer to entity types simply as entities. For example the CUSTOMER entity type would be referred to simply as the CUSTOMER entity. This is so common it would be inadvisable to do anything else, but technically an entity is an abstract instance of an entity type, and this is what an ER diagram shows - abstract instances, and the relationships between them. This is why entities are always named using singular nouns.

Mermaid can render ER diagrams

erDiagram CUSTOMER ||--o{ ORDER : places ORDER ||--|{ LINE-ITEM : contains CUSTOMER }|..|{ DELIVERY-ADDRESS : uses

Entity names are often capitalised, although there is no accepted standard on this, and it is not required in Mermaid.

Relationships between entities are represented by lines with end markers representing cardinality. Mermaid uses the most popular crow’s foot notation. The crow’s foot intuitively conveys the possibility of many instances of the entity that it connects to.

ER diagrams can be used for various purposes, ranging from abstract logical models devoid of any implementation details, through to physical models of relational database tables. It can be useful to include attribute definitions on ER diagrams to aid comprehension of the purpose and meaning of entities. These do not necessarily need to be exhaustive; often a small subset of attributes is enough. Mermaid allows to be defined in terms of their type and name.

erDiagram CUSTOMER ||--o{ ORDER : places CUSTOMER { string name string custNumber string sector } ORDER ||--|{ LINE-ITEM : contains ORDER { int orderNumber string deliveryAddress } LINE-ITEM { string productCode int quantity float pricePerUnit }

When including attributes on ER diagrams, you must decide whether to include foreign keys as attributes. This probably depends on how closely you are trying to represent relational table structures. If your diagram is a logical model which is not meant to imply a relational implementation, then it is better to leave these out because the associative relationships already convey the way that entities are associated. For example, a JSON data structure can implement a one-to-many relationship without the need for foreign key properties, using arrays. Similarly an object-oriented programming language may use pointers or references to collections. Even for models that are intended for relational implementation, you might decide that inclusion of foreign key attributes duplicates information already portrayed by the relationships, and does not add meaning to entities. Ultimately, it’s your choice.

学習者ノート

ここまでが ER 図が何であるかについての記述だ。名前は大文字で記すことや、単数形名詞であることなどを語っている。

Syntax

Entities and Relationships

Mermaid syntax for ER diagrams is compatible with PlantUML, with an extension to label the relationship. Each statement consists of the following parts:

<first-entity> [<relationship> <second-entity> : <relationship-label>]

Where:

• first-entity is the name of an entity. Names must begin with an alphabetic character and may also contain digits, hyphens, and underscores.

• relationship describes the way that both entities inter-relate. See below.

• second-entity is the name of the other entity.

• relationship-label describes the relationship from the perspective of the first entity.

学習者ノート

ER 図に関しては PlantUML 用に書いた図式を Mermaid で描画できるということになる。

For example:

PROPERTY ||--|{ ROOM : contains

This statement can be read as a property contains one or more rooms, and a room is part of one and only one property. You can see that the label here is from the first entity’s perspective: a property contains a room, but a room does not contain a property. When considered from the perspective of the second entity, the equivalent label is usually very easy to infer. (Some ER diagrams label relationships from both perspectives, but this is not supported here, and is usually superfluous).

学習者ノート

上記の関係 contains は対称律を満たさないと解釈するのだ。

Only the first-entity part of a statement is mandatory. This makes it possible to show an entity with no relationships, which can be useful during iterative construction of diagrams. If any other parts of a statement are specified, then all parts are mandatory.

Relationship Syntax

The relationship part of each statement can be broken down into three sub-components:

• the cardinality of the first entity with respect to the second,

• whether the relationship confers identity on a ‘child’ entity

• the cardinality of the second entity with respect to the first

Cardinality is a property that describes how many elements of another entity can be related to the entity in question. In the above example a PROPERTY can have one or more ROOM instances associated to it, whereas a ROOM can only be associated with one PROPERTY. In each cardinality marker there are two haracters. The outermost character represents a maximum value, and the innermost character represents a minimum value. The table below summarises possible cardinalities.

Value (left)	Value (right)	Meaning
|o	o|	Zero or one
||	||	Exactly one
}o	o{	Zero or more (no upper limit)
}|	|{	One or more (no upper limit)

学習者ノート

UML と比較すると、この多重度の表記法はピンと来ない。

Identification

Relationships may be classified as either identifying or non-identifying and these are rendered with either solid or dashed lines respectively. This is relevant when one of the entities in question can not have independent existence without the other. For example a firm that insures people to drive cars might need to store data on NAMED-DRIVER s. In modelling this we might start out by observing that a CAR can be driven by many PERSON instances, and a PERSON can drive many CAR s - both entities can exist without the other, so this is a non-identifying relationship that we might specify in Mermaid as: PERSON}|..|{CAR : "driver". Note the two dots in the middle of the relationship that will result in a dashed line being drawn between the two entities. But when this many-to-many relationship is resolved into two one-to-many relationships, we observe that a NAMED-DRIVER cannot exist without both a PERSON and a CAR - the relationships become identifying and would be specified using hyphens, which translate to a solid line:

erDiagram CAR ||--o{ NAMED-DRIVER : allows PERSON ||--o{ NAMED-DRIVER : is

学習者ノート

関係の分類について重要なことを説明している。実線で描かれている関係は、両側の実体は同時にしか存在しないと解釈できる。

Attributes

Attributes can be defined for entities by specifying the entity name followed by a block containing multiple type name pairs, where a block is delimited by an opening { and a closing }. For example:

erDiagram CAR ||--o{ NAMED-DRIVER : allows CAR { string registrationNumber string make string model } PERSON ||--o{ NAMED-DRIVER : is PERSON { string firstName string lastName int age }

The attributes are rendered inside the entity boxes.

The type and name values must begin with an alphabetic character and may contain digits, hyphens or underscores. Other than that, there are no restrictions, and there is no implicit set of valid data types.

学習者ノート

RDB におけるテーブル設計を意識した記法だ。

Attribute Keys and Comments

Attributes may also have a key or comment defined. Keys can be “PK” or “FK”, for Primary Key or Foreign Key. And a comment is defined by double quotes at the end of an attribute. Comments themselves cannot have double-quote characters in them.

erDiagram CAR ||--o{ NAMED-DRIVER : allows CAR { string allowedDriver FK "The license of the allowed driver" string registrationNumber string make string model } PERSON ||--o{ NAMED-DRIVER : is PERSON { string driversLicense PK "The license #" string firstName string lastName int age }

学習者ノート

こちらも RDB のテーブルを意識した記法だ。

Other Things

• If you want the relationship label to be more than one word, you must use double quotes around the phrase

• If you don’t want a label at all on a relationship, you must use an empty double-quoted string

学習者ノート

ラベル文字列に関する、わりと自明な制限だ。

Styling

Config options

For simple color customization:

Name	Used as
fill	Background color of an entity or attribute
stroke	Border color of an entity or attribute, line color of a relationship

Classes used

The following CSS class selectors are available for richer styling:

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

学習者ノート

Wikipedia の ER 図のページにあるような図式を表現するときにこれらを利用する。