sql - Can a superkey include things that aren't part of the primary key? -


Can things that are not part of the primary key?

logically speaking, if the column X in column < Code> {A, B, C} and A are the primary keys, then {A} , {A, B} {A, C} and {A, B, C} are all superkeys because in those sets you have all Know the value in the line (if it exists.)

However, for some purposes it is not considered as a key in SQL, e.g. If the table is y in A and b , then you no usually enter a foreign key Y A, B) References X (A, B) , because {A, B} is not the primary key if you want to be able to declare that foreign key, then you UNIQUE constraint has to be added to X (A, B) which is disabled as it duplicates the part of the primary key

In my opinion, this SQL In many flaws A.


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