together i recognize optionality method the minimum cardinality that a connection which is denoted as optional to optional, mandatory come optional, mandatory come mandatory..

And joining denoted together bold line and also a regular line.

In the web some express participation together the dependency of the entity to the partnership which is likewise looks like identifying and also non identify relationship.

You are watching: Participation in a relationship may be optional or mandatory.

and some refer it as the minimum cardinality

What is the correct definitions of those relationships and what is the difference..

inquiry Jun 26 "16 in ~ 0:07

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Let"s start with definitions and also examples of each of the concepts:

Total and partial participation:

Total authorized (indicated by a double or thick combination line) method that all the reality in an entity collection must participate in the relationship. Partial participation (indicated by a single thin line) method that there have the right to be entities in the entity collection that don"t take part in the relationship.


Medicine participates entirely in the develop relationship, an interpretation that medicine can"t exist unless developed by a Laboratory. In contrast, a Laboratory have the right to exist without creating Medicine - laboratory participates partly in the develop relationsip.

Mandatory and also optional roles:

In a relationship, roles have the right to be optional or mandatory. This affects whether a partnership instance can exist there is no an entity in a given role. Mandatory functions are shown with a solid association line, optional functions are suggested with a dotted line.


Roles aren"t often talked about in database tutorials, yet they"re an important concept. Think about a marriage - a partnership with two mandatory functions filled through the same entity set. In most relationships, the entity sets likewise define the roles, yet when an entity collection appears multiple times in a single relationship, we distinguish them in different roles.

In the instance above, a Patient have the right to Purchase medication with or without a Prescription. A acquisition can"t exist without a Patient and also Medicine, but a Prescription is optional (overall, despite it may be forced in certain cases).

Identifying relationship / weak entity:

A weak reality is an entity that can"t be figured out by its own attributes and therefore has another entity"s an essential as part of the own. One identifying connection is the relationship in between a weak entity and its parental entity. Both the identify relationship and also the weak reality are shown with double borders. Weak entity sets have to necessarily participate completely in their identifying relationship.


In this example, a Prescription consists of LineItems i beg your pardon are established by the Prescription"s key and a line number. In other words, the LineItems table will have a composite key (Prescription_ID, Line_Number).

For instances of non-identifying relationships, see the vault examples. While medicine participates entirely in the produce relationship, it has actually its own identification (e.g. A surrogate key, though ns didn"t indicate it). Keep in mind that surrogate keys always imply continual entities.

Mandatory/optional vs total/partial participation

Mandatory or optional roles show whether a certain duty (with its linked entity set) is compelled for the relationship to exist. Full or partial participation indicate whether a details relationship is required for an entity to exist.

Mandatory partial participation: view above: A Laboratory deserve to exist without producing any kind of medicine, but Medicine can"t be developed without a Laboratory.

Mandatory total participation: watch above: medication can"t exist without being Produced, and a activities can"t develop something unspecified.

Optional partial participation: see above: A Prescription have the right to exist without being Purchased, and also a Purchase have the right to exist without a Prescription.

That pipeline optional complete participation, which I had actually to think around a little bit to discover an example:


Some Patients die of one unknown Cause, however a cause of death can"t exist without a Patient dice of it.

Total/partial authorized vs identifying/non-identifying relationships

As I claimed before, weak reality sets always participate completely in their identifying relationship. View above: a LineItem have to be had in a Prescription, it"s identity and existence depends on that. Partial authorized in one identifying connection isn"t possible.

Total authorized doesn"t imply an identifying connection - medicine can"t exist there is no being produced by a Laboratory but Medicine is figured out by its own attributes.

Partial authorized in a non-identifying relationship is very common. For example, Medicine can exist without gift Purchased, and Medicine is established by its own attributes.

Mandatory/optional vs identifying/non-identifying relationships

It"s unusual for a connection to have less than 2 mandatory roles. Identify relationships are binary relationships, for this reason the parent and also child roles will be mandatory - the contain relationship between Prescription and LineItem can"t exist without both entities.

Optional roles are normally only uncovered on ternary and higher relationships (though view the example of patients dying of causes), and aren"t involved in identification. An different to an optional role is a partnership on a relationship:


By turning Purchase right into an associative entity, we can have it take part in a Fill relationship with Prescription. To maintain the exact same semantics as over I stated that a Purchase deserve to only to fill one Prescription.

Physical modeling

If we interpret from conceptual to physical model (skipping reasonable modeling / more normalization), making different tables for each entity and relationship, points look quite similar, despite you have to know how to review the cardinality indications on the foreign vital lines to recover the ER semantics.

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However, it"s usual to denormalize tables with the same main keys, an interpretation one-to-many relationships are linked with the entity table top top the many side:


A relationship is physically represented as two or more entity keys in a table. In this case, the entity secrets - patient_id and also cause_of_death_id are both uncovered in the patient table. Many civilization think the foreign key line represents the relationship, however this comes from confusing the entity-relationship version with the old network data model.

This is a critical point - in order to understand various kinds that relationships and constraints ~ above relationships, it"s vital to know what relationships space first. Relationships in ER room associations between keys, not between tables. A relationship can have any number of roles of various entity sets, when foreign key constraints enforce a subset constraint between two columns the one reality set. Now, armed with this knowledge, read my entirety answer again. ;)