Describe All-key Relations in Database

In database design, "all-key relations" refer to a specific type of table or relation where every attribute (or column) is part of the primary key. The purpose of all-key relations is closely tied to the principles of normalization, particularly in the context of advanced normalization forms such as the Fourth Normal Form (4NF) and Fifth Normal Form (5NF).
The primary purposes of all-key relations in database design include:

1. Eliminating Redundancies: By ensuring that every attribute in a relation is part of the primary key, all-key relations help in eliminating redundancy. This is because no non-key information is stored, and every row represents a unique combination of values, reducing the likelihood of duplicate data.
2. Enhancing Data Integrity: All-key relations inherently enforce a higher degree of data integrity. Since all attributes are part of the primary key, each tuple (row) in the relation is guaranteed to be unique. This uniqueness constraint helps in maintaining accurate and consistent data, which is fundamental for reliable database systems.
3. Supporting Advanced Normalization: All-key relations are often the result of decomposing tables to achieve higher normal forms such as 4NF and 5NF, which are aimed at reducing d re not implied by candidate keys, often resulting in all-key relations.
4. Facilitating Efficient Data Retrieval: Since all-key relations reduce redundancy and enhance data integrity, they can lead to more efficient data retrieval operations. The database can perform queries and join operations more efficiently when the data is well-structured and normalized, leading to faster response times for complex queries.
5. Improving Scalability: All-key relations can contribute to better scalability of the database system. By minimizing redundancy and ensuring that each piece of data is stored only once, it reduces the storage requirements and improves the performance of the database as it scales.
6. Enhancing Flexibility for Querying: With all-key relations, querying and data manipulation can become more flexible. Since each row is uniquely identified by its key attributes, it becomes easier to query specific data points without ambiguity.

It's important to note that while all-key relations have several benefits, they are not always the best or most practical solution in every scenario. The decision to use all-key relations should be balanced with considerations of query performance, ease of understanding for database users, and the specific requirements of the application or system being designed.

When a table is created to represent an entity, it is possible that every column in the table will be part of the entity's primary key. As an example, examine the entities in the following graphic.

The entity named AVAILABLE_IN contains two columns, both of which are part of the entity's primary key. When every field in a relation is part of the primary key for that relation, the relation is referred to as an all-key relation ^[1]. All-key relations are valuable because they enable you to create many-to-many relationships between tables in the database without incurring the problems of many-to-many relationships.
AVAILABLE_IN provides just such a bridge between the ITEM and LANGUAGES tables. Rather than including fields in the ITEM table to record in which languages the CD is available (which is bad design, as we will see), or creating a many-to-many relationship (which is also bad design), users can enter as many languages as they need to describe each Stories on CD product.
Note: The entity AVAILABLE_IN contains the column ItemNo which is a foreign key to the table ITEM. The information about languages is stored in the AVAILABLE_IN table.
The next lesson describes foreign keys^[2].

It is possible to have a table in which every column is part of the primary key. As an example, consider a library book catalog where each book title owned by a library has a natural unique primary key: the ISBN (International Standard Book Number). Each ISBN is assigned to one or more subject headings in the library's catalog, and each subject heading is also assigned to one or more books. We therefore have a many-to-many relationship between books and subject headings. A relation to represent this relationship might be:

subject_catalog (isbn, subject_heading)

All we need to do is pair a subject heading with a book identifier and no additional data is needed. Therefore, all columns in the table become part of the primary key. There is absolutely no problem with having all-key relations in a database. In fact, they occur whenever a database design contains a composite entity that has no relationship data. They are not necessarily errors, and you can use them wherever needed.

Key constraints include primary keys, foreign keys, and unique keys. All these key types are discussed briefly later on in this chapter and further in later chapters in this book. Key constraints allow the checking and validation of values between fields in different tables. Primary and foreign keys are essentially the implementation of relationships between parent and child tables. Those relationships or relations are the source of the term relational database.

[1] all-key relation: A relation in which every field is a member of the relation's primary key

[2] foreign key: A field (or combination of fields) used to link tables; a corresponding primary key field occurs in the same database.