I was going through some exercise problems to check my understanding of the relational model. According to my understanding, if we are given Album[mid] ⊆ Musician[mid], we can translate it into plain English as “Every album in the Database must have at least one musician”. If my understanding is right, How can the answer be “atmost” as given in the answers?
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Answer
The symbol “⊆” means “is a subset of”. In Album, mID is a foreign key to Musician’s primary key mID. For each and every given Album (aID)primary key there can only be one value for mID (not two or more), which points to a single row of Musician (mID) primary key. A primary key value should always be unique, else its not the 4th database normal form.