Section 5 Flashcards
A nontransferable relationship is represented with the ……….. on the relationship
diamond
Nontransferable relationships are important to note because they usually reflect a ………… ………. and will have implications when the ERD is implemented as a ………………..
business rule / database
The diamond is typically drawn on the “………….” (……………….) side of the relationship.
many / crows-foot
The various types of 1:M relationships are ……….. ………….. in an ER Model
most common
“Many” can mean ………….. or ………….., depending on the optionality.
one-or-more / zero-or-more
Mandatory at both ends: This type of relationship typically models entities that cannot exist without each other. This usually represents an ideal situation—we cannot have ORDER ITEMs without ORDERs.
R 2
Mandatory on the one side, optional on the many side: This is rarely used. You will see it only when the relationship expresses that an entity instance exists only when it is a ………… …………, and where the elements of the set can exist ……………….
nonempty set / independently.
A MUSICIAN may be part of one BAND. A BAND is of no interest if it is empty. How can you have a BAND without MUSICIANs?
The various types of M:M relationships are common, particularly in a ……….. ……….. of an ER model. In later stages of the modeling process, all M:M relationships will be …………, and …………
first version
resolved / disappear
In most cases, M:M relationships indicate that an entity is missing from the model.
Usually you will find just a few of the various types of ……….. relationships in every ER model
1:1
Mandatory at one end of the 1:1 relationship commonly occurs when roles are ……………
modeled
Alternatively, TEACHER and STUDENT could have been modeled as subtypes of PERSON, unless a PERSON can be both a TEACHER and a STUDENT at the same time.
1:1 relationships (of all three variations) also occur when some of the entities represent various stages in a …………….
process
Mandatory 1:1 A 1:1 relationship, mandatory at both ends, tightly connects two entities: when you create an instance of one entity, there must be exactly one dedicated instance for the other simultaneously. This leads to the question why you want to make a distinction between the two entities anyway. The only acceptable answer is: ………………………………..
only if there is a business need.
A redundant relationship can be derived from …………. ………… in the model
another relationship
In this example, you can derive the relationship from PERSON to COUNTRY from the other two relationships (COUNTRY to TOWN, TOWN to PERSON), so you should remove the direct relationship from COUNTRY to PERSON
However, be careful of concluding that a relationship is redundant based on the structure alone. Read the …………….. to check
relationships
- The ERD shown here does not reflect a redundant relationship*
- The relationship on the right tells us the country of a PERSONs birth, and the relationship on the left tells us the TOWN and COUNTRY that a person currently lives in. It is possible that someone could be living in a different COUNTRY from where they were born.*
What is the solution?
A third entity is needed to resolve the M:M relationship. This is called an “intersection” entity
What would be the UID of the intersection entity?
