You can (but don't have to) specify the attributes’ types. Like entities, associations have names (displayed at the top) and attributes (displayed in the lower part of the rounded rectangle). Or you can implement a ternary relationship, which cannot be achieved through a simple relationship between entities.Īssociations are represented in a diagram by a rectangle with rounded corners. In this way, you can realize a many-to-many relationship with attributes. AssociationĪn association is a special representation of a relationship between two entities – specifically, a relationship with additional attributes. Using these nonstandard symbols can make a programmer’s life a lot easier. association and inheritance) you can use when creating your logical ER diagram. That's why Vertabelo also supports some nonstandard functions (e.g. If you’re dealing with complicated databases, the above notations may not be enough. Nonstandard Logical ER Diagram Symbols Used in Vertabelo You can read more about many-to-many relationships here. So the school entity isn't mandatory, which we denote by an empty circle. On the other hand, a teacher can be currently unemployed but still in the database. The teacher entity is mandatory – each school must have some teachers. In this kind of relationship, there are crow’s feet at both ends. Each school has many teachers and the teachers can teach in a few different schools. Each student must be assigned to a school and each school must have some students. Each school has many students (hence the crow’s foot near the students entity) but each student is only in one school.Įach entity is mandatory (hence the vertical line). This is shown by the vertical bars near each entity.Ībove, we see a many-to-one relationship. Each principal is a principal in only one school and each school has only one principal, so there are no crow’s feet.Įach entity is mandatory – each school needs to have a principal and there is no principal without the school. If it's not mandatory, we denote it as an open circle. If the entity is mandatory, we denote it by a vertical segment. The many side is denoted by the crow’s foot symbol one is denoted by a single line.Įach entity in a relationship can be mandatory or optional. Relationships can be one-to-one, one-to-many, or many-to-many. Each relationship is one line and has cardinality and mandatory attributes. These relationships are denoted in diagrams by lines. You can also specify attributes’ data types, as shown below: RelationshipsĪfter creating entities, you may need to specify the relationships between them. You use this for the attribute(s) that uniquely identify the whole entity. The PI annotation stands for "primary identifier". In other words, M attributes must have a value. The M annotation next to the attribute stands for "mandatory" – you use it for attributes that can't be left empty. Here's an example of an entity in Vertabelo: You can list the attributes with or without their types. This rectangle is divided into two parts: the entity name at the top and the entity attributes in the lower part. When creating a logical ER diagram, you typically use the following symbols: Entity Standard Symbols Used in Logical ER Diagrams There are a few standard symbols used in logical and physical ER diagrams, and some useful additional non-standard symbols that you can use in Vertabelo. This is the default ER diagram notation used in Vertabelo. The most popular notation in ER diagrams is the Information Engineering (IE) notation, also called crow’s foot notation.
Find out what symbols are used in the Entity-Relationship Diagram (ERD) and what they mean.
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