ER Diagram Symbols and Their Usage
ERD
When you need to create an ER diagram to document a database, it will be much easier using pre-made symbols and icons. This page gathers a large quantity of useful symbols that often used in ER diagrams, Chen ERD, Express-G diagram, ORM diagram, Martin ERD and database model diagram.
Entity Relationship Symbols
Below are pre-drawn entity relationship symbols in Edraw ER diagram software, including entity, view, parent to category, category to child, dynamic connector, line connector, strong entity, weak entity, strong relationship, weak relationship, attribute, derived attribute, constraint and participation, etc.
Entity in a database could be a single person, place, or thing about which data can be stored.
Relationship provides useful information that could not be discerned with just the entity types. Relationships are meaningful associations between or among entities like assigning, associating, or tracking.
Categories are tables that are subtypes of a parent table. In relation to a database , an entity is a single person, place, or thing about which data can be stored.
Strong entity is one that exists on its own, independent of other entities.
Weak entity is one whose existence depends on another entity. A weak entity cannot be uniquely identified by its attributes alone.
Strong relationships are connections that exist between a strong entity type and its owner.
Weak relationships are connections that exist between a weak entity type and its owner.
Attribute is a piece of information which determines the properties of a field or tag in a database or a string of characters in a display.
When you use the category symbols, you may notice that there is a floating button on the shape. Click the floating button, you may set the property of the category.
Some people may be confused how to use connector to connect entity symbols. Actually, it is easy, you can move the connector towards the shape, then you can find that the entity shape has lots of green marks. when the red mark appear, it means the two has connected.
Object Relationship Symbols
The following picture is the object relationship symbols including entity, relationship, view, parent to category, category to child, table inheritance, type, type inheritance and dynamic connector.
Type of a relationship is created depends upon how related objects are defined.
Type inheritance is an additional property offered by hierarchy within structured complex data.
Chen ERD Symbols
Entity in a database could be a single person, place, or thing about which data can be stored.
Attribute is a piece of information which determines the properties of a field or tag in a database or a string of characters in a display.
Aggregation is a special type of association in which objects are assembled or configured together to create a more complex object. An aggregation describes a group of objects and how you interact with them.
One to many (N) is by far the most common relationship type.
Many to many (M) lets you relate each row in one table to many rows in another table, and vice versa. Many-to-many relationships often hide meaning.
Relationship Symbols
Composition relationship, which was named composition association relationship in UML 1.4, represents whole-part relationships and is a form of aggregation. A composition relationship specifies that the lifetime of the part classifier is dependent on the lifetime of the whole classifier.
Inheritance relationship helps to communicate structure and inheritance of an object model. Inheritance relationships organize classes into generalization-specialization (superclass-subclass) hierarchies; they provide a basic re-use mechanism for sharing attributes and operations.
Dependency relationship is a relationship in which one element, the client, uses or depends on another element, the supplier.
Association is a relationship between two classifiers, such as classes or use cases, that describes the reasons for the relationship and the rules that govern the relationship.
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Express-G Symbols
Entity in a database could be a single person, place, or thing about which data can be stored.
Defined type is the most important datatype in Express-G.
Select type defines a choice or an alternative between different options. Most commonly used are selects between different entity types.
Tree structure is a structure in which there are successive branchings or subdivisions.
Schema is a way to logically group objects such as tables, views, stored procedures, etc.
Page reference is a reference to an instantiation of a page.
Martin ERD Symbols
Data entity is an object, event, or phenomenon about which data is stored in a database.
Data item is a unit of data contained in a record, describing a particular attribute, and it requires one or several bits, bytes, or words to represent an entity.
Connection is the setting up of resources so that a particular object such as a database or file can be read or written to.
Events with different colors represent different events.
ORM Diagram Symbols
Entity in a database could be a single person, place, or thing about which data can be stored.
Unary means consisting of or involving a single component or element.
Ternary means "composed of three items".
Index annotation defines an index for the class as a whole (typically a composite index).
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Learn about Entity Relationship Diagram ERD