Sep 13, The Optionality is a property of an attribute which specify if a value is mandatory Often, this rule is applied with a NOT NULL clause in the database. Entity Integrity (Uniqueness|No Duplicate|Distinct) · Entity-Relationship. It can be useful to know one additional piece of information about a relationship. This is what's called the relationship's optionality. This information is not strictly. Nov 30, (Remember - 'entity' and 'instance of an entity' are not the same thing. There are 3 types of cardinality for binary and unary relationships.
Note that surrogate keys always imply regular entities.
Total or partial participation indicate whether a certain relationship is required for an entity to exist. A Laboratory can exist without producing any medicine, but Medicine can't be Produced without a Laboratory.
Medicine can't exist without being Produced, and a Laboratory can't Produce something unspecified. A Prescription can exist without being Purchased, and a Purchase can exist without a Prescription.
That leaves optional total participation, which I had to think about a bit to find an example: Partial participation in an identifying relationship isn't possible. Total participation doesn't imply an identifying relationship - Medicine can't exist without being Produced by a Laboratory but Medicine is identified by its own attributes.
Partial participation in a non-identifying relationship is very common. For example, Medicine can exist without being Purchased, and Medicine is identified by its own attributes. Identifying relationships are binary relationships, so the parent and child roles will be mandatory - the Contain relationship between Prescription and LineItem can't exist without both entities. Optional roles are usually only found on ternary and higher relationships though see the example of patients dying of causesand aren't involved in identification.
An alternative to an optional role is a relationship on a relationship: By turning Purchase into an associative entity, we can have it participate in a Fill relationship with Prescription.
To maintain the same semantics as above I specified that a Purchase can only Fill one Prescription. However, it's common to denormalize tables with the same primary keys, meaning one-to-many relationships are combined with the entity table on the many side: A relationship is physically represented as two or more entity keys in a table.
Many people think the foreign key line represents the relationship, but this comes from confusing the entity-relationship model with the old network data model.
This is a crucial point - in order to understand different kinds of relationships and constraints on relationships, it's essential to understand what relationships are first. Relationships in ER are associations between keys, not between tables.
A relationship can have any number of roles of different entity sets, while foreign key constraints enforce a subset constraint between two columns of one entity set.
Data Modeling - Optionality (NOT NULL)
Now, armed with this knowledge, read my whole answer again. For example, Each students take many modules, each module is taken by many students. How many is many?
If it's 0, 10 orthe way you implement the relationship is the same. Each relationship can be optional or mandatory for each entity. This gives three types for binary relationships: Optional for both entities Optional for one entity, mandatory for the other Mandatory for both entities Putting all of this together there are 6 binary relationships that we need to know how to implement: It is because the many that it is associated with could be 0, 10, etc.
Data Modeling - Optionality (NOT NULL) [Gerardnico]
The point is the 0. The way we implement the relationship means that the one side could be associated with zero on the many side - it's got optioanlity built in.
This will become clearer when we start resolving relationships. For now, the important thing is the meaning of the three terms, degree, cardinality and optionality.
These are dictated by the 'Enterprise rules'. Nor does it matter what happens outside of our database design. The enterprise rules, rule. For example, an enterprise rule might be: An employee must have one address, each address can house more than one employee. But, you say, an employee could have a second home. What matters is the enterprise rule.