Does Every SQL Table Possess a Superkey?

Do all SQL tables have super keys?

In the world of relational databases, a superkey is a set of attributes that uniquely identify each row in a table. However, SQL tables are not truly relational models, which begs the question: Do SQL tables have superkeys?

Define superkey in SQL context

To understand superkeys in SQL, we need to consider the difference between mathematical relationships and SQL tables. In a mathematical relationship, each row is unique and there are no duplicate values. However, SQL tables allow duplicates and may contain null values, which complicates the definition of superkeys.

SQL superkey can be loosely defined as a set of columns that can be declared as a primary key or a unique non-null constraint. Additionally, an empty table can treat each attribute subset as a superkey, while a table with no duplicates and null values ​​can be considered a relationship with a superkey.

Empty SQL table

Empty SQL tables have no rows, so they can be thought of as having a subset of each attribute as a superkey. This is because any set of attributes will uniquely identify a row that does not exist.

All SQL tables

Every SQL table has at least one superkey, regardless of whether it contains duplicates or null values. This is because the set of all columns in a table is always a superkey. Even though duplicates and nulls might prevent the table from being a purely mathematical relationship, it still has a superkey that identifies the rows.

Conclusion

While the concept of superkeys originates from relational theory, it is informally applied to SQL tables. Despite their differences from mathematical relationships, SQL tables can legitimately be considered to have superkeys, either as primary keys or unique non-null constraints, or as a set of all columns. However, it is important to remember that these superkeys may not be minimal or unique due to duplicates or null values.

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