The figure below describes a database about bands and their tours. A tour consis
ID: 3712228 • Letter: T
Question
The figure below describes a database about bands and their tours. A tour consists of a sequence of cities visited by a band. We assume that no city is visited twice on a single tour, and on one date, a band can visit only one city. Some of the entity sets are weak, and some of the relationships are supporting many-one relationships, but all double rectangles and double diamonds are not shown. 1. Your task is to decide which of the entity sets are weak, and which relationships support them. If there is a choice, prefer to use the attributes of the entity set itself in the key, and minimize the number of supporting relationships. There may be more than one reasonable answer. especially since "tour#" can have several reasonable interpretations city Name date bandName Cities At Stops Bands In On States Tours stateName ur NumberExplanation / Answer
CORRECT ANSWERS
Stops is weak, supported by On.
Tours is weak, supported by By; tour# is unique only for a given band.
Tours is not weak; tour# is a globally unique ID.
Cities is weak, and its key is the city name and the name of the state it is In.
INCORRECT ANSWERS
Stops is weak, supported by both At and On; date is not a key attribute.
– We are told that a tour cannot stop at two cities on the same date. Thus, once we know the date and the tour that a Stop represents, we don't also need the City to have a key for Stops.
Stops is weak, supported by both At and On; date is a key attribute.
– We are told that a tour cannot stop at two cities on the same date. Thus, once we know the date and the tour that a Stop represents, we don't also need the City to have a key for Stops.
Stops is weak, supported by At.
– A date and a city does not define a unique Stop (City/Tour pair). The reason is that several tours may stop at the same city on the same date.
Cities is not weak.
– The city name by itself does not determine a unique city. There can be cities of the same name in two or more different states, e.g., Portland ME and Portland OR.
Bands is weak, supported by By.
– A supporting relationship has to be many?one from the entity set it supports. In order for this diagram to make sense, we have to assume that band names are unique, because By cannot help define a band uniquely using the weak entity set construct.
States is weak, supported by In.
– A supporting relationship has to be many?one from the entity set it supports. Note also that state names are unique.
Stops is weak, and its key is the date and the name of the band making the tour the stop represents.
– There is no many?one relationship directly from Stops to Bands. Thus, an attribute of Bands can only contribute to the key of Stops if Tours is weak and the band name constributes to the key of Tours. But then, the tour# would also be in the key for Stops.
Stops is weak, and its key is the date, the name of the associated city, and the associated tour#.
– Since it is ambiguous whether tour#'s are unique, it is possible that there are two bands that use the same tour number, and these bands stop in the same city on the same date.
Tours is weak, supported by On and By.
– A supporting relationship has to be many?one from the entity set it supports. Thus, On couldn't possibly support Tours.
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.