How To Draw Crow Foot Erd
Crow's Human foot Notation
A number of data modeling techniques are being used today. One of the most common is the entity relationship diagram (ERD). Several ERD notations are available. For the purposes of CS270 we will be using Crow'southward Foot Notation.
Components used in the creation of an ERD:
Entity – A person, place or thing about which nosotros want to collect and store multiple instances of information. It has a proper name, which is a substantive, and attributes which describe the data nosotros are interested in storing. It also has an identifier, which uniquely identifies one instance of an entity. The attribute which acts as the identifier is marked with an asterisk.
Relationship – Illustrates an association between 2 entities. It has a name which is a verb. It besides has cardinality and modality.
Cardinality and Modality are the indicators of the business rules around a relationship. Cardinality refers to the maximum number of times an instance in one entity tin be associated with instances in the related entity. Modality refers to the minimum number of times an case in 1 entity can exist associated with an instance in the related entity.
Cardinality can be 1 or Many and the symbol is placed on the outside ends of the human relationship line, closest to the entity, Modality can be i or 0 and the symbol is placed on the inside, side by side to the cardinality symbol. For a cardinality of i a direct line is fatigued. For a cardinality of Many a human foot with three toes is drawn. For a modality of 1 a straight line is drawn. For a modality of 0 a circumvolve is drawn.
Cardinality and modality are indicated at both ends of the relationship line. One time this has been done, the relationships are read as being 1 to one (one:one), 1 to many (1:M), or many to many (M:Yard).
1:ane
1:M
G:M
1:M
Typically, ERDs are much more than complex than this, involving quite a number of entities and relationships. If we bring together all of the to a higher place relationships together and add a few attributes, a pocket-size collection of data might be depicted in the post-obit way using Crow'due south Human foot Notation:
Consider the following business rules for a patient date system:
A dr. tin be scheduled for many appointments, only may not have any scheduled at all. Each appointment is scheduled with exactly 1 dr.. A patient tin schedule 1 or more appointments. Ane appointment is scheduled with exactly 1 patient. An engagement must generate exactly 1 bill, a neb is generated past only one appointment. I payment is applied to exactly 1 neb, and i bill can be paid off over time by several payments. A bill can exist outstanding, having nothing notwithstanding paid on it at all. One patient tin brand many payments, but a unmarried payment is made by only 1 patient. Some patients are insured past an insurance visitor. If they are insured, they tin only carry insurance with one company. An insurance compnay can have many patients deport their policies. For patients that behave insurance, the insurance visitor will make payments, each single payment is made past exactly 1 insurance company.
Given the higher up data, the post-obit ERD tin can be fatigued:
Intersection Entities
Intersection entities are used in the resolution of a many to many human relationship. This resolution is done in order to shop boosted data which doesn't fit into the attribute listing of either entity in the Yard:Thou relationship. For instance, in the instructor-course example given earlier, there is a Thou:M relationship between Grade and Section. A course can have many sections, and a section tin accept many courses. If we desire to know how many students are registered in a particular class, we cannot store the data on either entity. We could store the full number of students registered in all sections of a item course, in the course entity, ie: 300 students are registered in CS270, all sections included. We could store the full number of students registered in all courses with a item section number, in the section entity, ie: 25,000 students are registered in a department numbered 002. If we want more than practical information, such as how many students are registered in section 002 of CS270, nosotros demand another entity in which to store the information. So, we create an intersection entity, and place information technology between its related entities with the additional attributes. The One thousand:G relationship becomes two 1:M relationships. See resolution beneath.
Independent Entities
Independent entities are those which can be without another entity. Pupil, seat, instructor etc. are examples of independent entities.
Dependent Entities
Dependent entities rely upon the existence of other entities. Some of their attributes are taken from the identifiers in the entities upon which they depend. Class is an example of a dependent entity. It relies upon course and section, and gets two of its attributes from their identifiers.
Source: http://www2.cs.uregina.ca/~bernatja/crowsfoot.html
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