An entity is a thing capable of an existence. An entity usually refers to some aspect of the real world, which can be distinguished from other aspects of the real world (a person, place, customer transaction, order, etc.).
An entity is represented by a box shape on the diagram. An entity has two compartments where properties (columns) of the entity can be specified. The upper compartment holds the primary key properties of the entity; the lower compartment holds other properties.
A relationship between entities describes how entities are associated. Relationships are represented by lines, connecting entities. Relationship end adornments indicate multiplicities of these ends. Multiplicity is the number of entity instances that can be associated with a number of another entity instances. Relationship multiplicity is represented by three symbols, the so-called “crow's foot notation” or “Information Engineering notation”.
The following table shows symbols of the relationship multiplicity.
Multiplicity lower bounds and upper bounds are paired into one adornment, as shown in the following table. Note that any lower bound, which is more that 0 is treated as 1 (this also includes lower bounds greater than 1, such as 2.). Also, any upper bound which is greater than 1 is treated as Many (this also includes upper bounds less than unlimited, such as 7.).