> 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.
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 are a relatively new feature in Mermaid, so there are likely to be a few bugs and constraints, and enhancements will be made in due course. Currently you can only define entities and relationships, but not attributes. Inclusion of attributes is now actively being worked on.
Mermaid syntax for ER diagrams is compatible with PlantUML, with an extension to label the relationship. Each statement consists of the following parts:
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).
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.
- 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 characters. The outermost character represents a maximum value, and the innermost character represents a minimum value. The table below summarises possible cardinalities.
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:
