Do SQL Tables Always Contain Superkeys?

Does the SQL table contain superkeys?

This article explores the concept of superkeys in SQL tables and answers the following questions:

• Can an empty SQL table have a superkey?
• Does every SQL table inherently contain superkeys?

Relational Model and Superkeys in SQL

The term "superkey" originates from the relational model (RM), which describes a set of attributes that uniquely identify a row in a relationship. In an RM relationship, a superkey is a set of attributes such that the values ​​of these attributes are different for each different row.

However, SQL tables differ from RM relationships in some ways, including allowing duplicates and null values. Therefore, the concept of superkeys in SQL tables becomes subtle.

Super key in empty SQL table

An empty SQL table, containing no rows, presents a unique situation. According to the RM definition, for a null relation, each subset of attributes constitutes a superkey. This is because there are no rows to compare, and any subset of attributes becomes a valid identifier.

Super key in non-empty SQL table

For non-empty SQL tables that do not enforce uniqueness or allow null values, the concept of superkeys becomes questionable. Some interpretations of the RM definition suggest that any set of attributes that satisfy the uniqueness condition of a relationship can be considered a superkey.

However, it is important to note that SQL tables can contain duplicate rows and null values, which may challenge the concept of uniqueness. Therefore, defining superkeys in SQL tables requires a certain informality.

Informal view of super keys in SQL tables

In a more practical approach, we can define a SQL table superkey as a set of attributes that can be declared as a primary key or a unique non-null constraint. Additionally, empty superkeys (i.e. {}) can be included for tables containing at most one row.

Conclusion

Although the concept of superkeys is formally defined in the relational model, applying it to SQL tables requires a certain degree of adjustment. For an empty SQL table, each subset of attributes can be considered a superkey. For non-null tables, the interpretation of superkeys becomes more informal, focusing on sets of attributes that can enforce uniqueness or prevent duplicates and nulls.

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