Question

a. Suppose that a model of a music library does not model ‘artist’ as an entity set, but instead models it as an attribute of a track. This could lead to what the authors of our text call ‘incompleteness’. Why would the they call this model incomplete? Is it because a) not all artist information could be contained as an attribute of a track, b) it would not be possible to model an artist before that artist released any tracks, or c) the artist could not have a key.

b. Can one entity belong to two entity sets? (yes/no)

c. Is a table in a database more like a) an ER relationship or b) an ER relationship set?

d. When would it be wise to specify the role played by an entity in a relationship? a) when the same entity set participates more than once in the relationship set, b) when the relationship is ternary, or c) when the entity represents an abstract rather than physical thing.

e. In an ER model, would it be possible for the same two entity sets to participate in two different relationship sets? (yes/no)

f. It is possible for a attribute to be multivalued in a ___________________ but not a _________________. Should the blanks be filled with a) "relational schema", "ER model", or b) "ER model", "relational schema"?

g. In the music library example, a possible entity set is ‘playlist’. If the playlist gives the IDs of tracks, and each track has a ‘length’ attribute (giving how long the track is in seconds), then the length of the playlist could be calculated from the track lengths. So should playlist have an attribute ‘length’ in an ER model? a) yes, b) no, or c) the choice would be a modeling decision.

h. Why might you want to model ‘address’ as a composite attribute (built from attributes ‘street’, ‘city’, ‘state’, and ‘zip’, perhaps) rather than just using all of the individual attributes? Would it be a) because in the model we might want to sometimes refer to the address as a whole and sometimes to parts of the address, b) to save space, or c) to avoid possible null values.

i. Suppose a music library model includes entity sets ‘album’ and ‘artist’, and relationship set ‘album-artist’. In this model, would you expect the participation of album in album-artist to be ‘total’? (yes/no)

j. In an ER model, we could model the department of an instructor as using a 'department' entity set, or with a 'department_name' attribute of the 'instructor' entity set. Which is the best reason for choosing to model the department as an entity set? a) some instructors don't have departments, b) some instructors have multiple departments, or c) there is additional information about departments that we will probably want to model.

Answer 1

A. The ER model of the music library would be incomplete if artist is considered as an attribute. an attribute cannot be uniquely identified like an entity. Remember, Key is the attributre or set of attributes that uniquely identify an entity. The artist attribute could not have a key. So there is no way to uniuely identify a particular artist.
B. An entity set is a set of entities of the same type that share the same properties. Let's say there is an entity set ,say artist(as in this case )means it has a set of artists who share the same properties like singing, music making , acting etc. There cannot be another entity set with exactly same properties. In other words, there cannot be another entity set which has has different set of properties that the same entity might have. thus, answer is No.
C.Is a table in a database more like a) an ER relationship or b) an ER relationship set?
Table is more like a ER relationship set .Relationship set is a set of tuples that has atleast one value/component for each entity.
D.When an entity set participates more than once in the relationship set, the role name is explicitly specified instead of entity name. Example: if employee records info about all the employees in the bank.We may have a relationship set called works-for. Here, one will be worker and the former will be manager who gets info about the workers. Both are from employee entity set.
E. same two entity sets to participate in two different relationship sets? YES they would play different roles.
F.It is possible for a attribute to be multivalued in a ERmodel but not a relational schema.
In ER model, it is represented as double ovals.
G.should playlist have an attribute ‘length’ in an ER model? This would be a modelling decision. If there is a need, they can have the length attribute. If not a must for any other table, it can be omitted.
H.Address should be a composite attribute because in the model we might want to sometimes refer to the address as a whole and sometimes to parts of the address.

I. Yes the relationship would be total participation because , one artist will have atleast one album in the music library.
J. Yes we need to model the department as an entity set because there is additional information about departments that we will probably want to model. i.e. it can have some more attributes other than instructor.