Do All SQL Tables Have Superkeys?

Super key in SQL table

In relational database theory, a super key refers to a set of attributes that uniquely identify each row in a relationship (table). Unlike relationships, SQL tables allow duplicate rows and null values, which makes the concept of superkeys less straightforward in this context.

Can an empty SQL table have super keys?

The answer to this question lies in the definition of a superkey: a set of attributes that uniquely identify each row. Since an empty table contains no rows, the need for unique identification becomes irrelevant. Therefore, every subset of attributes in an empty SQL table qualifies as a superkey.

Does every SQL table always have a superkey?

The definition of superkey emphasizes that there are no duplicate rows. However, SQL tables can contain duplicates and null values. This raises the question: Does every SQL table have a superkey that meets uniqueness requirements?

The answer lies in that a SQL table can be interpreted as a relation if certain conditions are met: there are no duplicate rows, there are no null values ​​in the columns participating in the superkey formation, and there is at least one row (satisfying the requirements of the relation involving non-empty tuples definition).

If a SQL table satisfies these conditions, it can be considered a relationship, and it can be inferred that it holds one or more superkeys that uniquely identify each row.

In summary, although every empty SQL table has a superkey, not all SQL tables necessarily have a superkey that meets uniqueness requirements because there may be duplicate rows and null values. Only if a SQL table meets the conditions to be considered a relationship can we confidently assert that it holds one or more superkeys.

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