From the specification, "Books are written by authors who specialise in one or more subjects". An author can therefore write in more than one subject (one to many), and more than one author can write in a single subject (many to one). Combined together, this is many to many, which is what appears in the ER diagram.
From the specification, "...the contribution of each author is recorded..." An author can therefore write many contributions covering one or more publications (one to many), but a single contribution only concerns a single author (one to one). Combined together, this makes one to many, which is what appears in the ER diagram.
From the specification, "A publication covers a single subject area..." A publication is thus in a single subject area (one to one), but more than one publication can exist in each subject area (many to one). Combined together, this is many to one, which is what appears in the ER diagram.
From the specification, "...the contribution [to a publication] of each author is recorded...". Thus there can be many contributions for a single publication (many to one), but a single contribution only concerns a single publication (one to one). Combined together, this is many to one, which is what appears in the ER diagram.
From the specification, "...[Editors] take sole responsibilty for for editing one or more publications...". Thus an editor can edit more than one publication (one to many), but each publication has only a single editor. Thus the relationship for "edits" should be one to many, not many to many.
A publishing company produces academic books on various subjects. Books are written by authors who specialise in one or more particular subject. The company employs a number of editors who do not have particular specialisations but who take sole responsibilty for for editing one or more publications. A publication covers a single subject area but may be written by one or more author - the contribution of each author is recorded as a percentage for the purposes of calculating royalties.
The following ER diagram is intended to represent the above specification:
Indicate the relation which has an incorrect cardinality shown:
- specialises in
- makes
- is about
- to
- None of the above
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