CODEWITHSUNDEEP
databasedatabase-designrelational-database
user1002563
user1002563

Reputation: 335

Why are relational sets important?

A friend is developing a website, and has to make a database using SQL. He asks why do you need "has-a" or "is-a" relationships since you can take the primary keys of a one entity set and place it in the other appropriate entity set (and vice-versa) to find the relations.

I could not answer the question because I was just taught that relational sets are just how database works.

Edit: I did not want to go into normalization. He made a point that the information is replicated in the relationship set.

Upvotes: 0

Views: 111

Answers (1)

Walter Mitty
Walter Mitty

Reputation: 18950

Your question mixes two different levels of abstraction together, namely the conceptual level and the logical level.

At the conceptual level, one is interested in describing the information requirements on the proposed database. It's useful to do this without tilting the description towards one solution or another. One model that is useful for this purpose is the Entity-Relationship (ER) model. In this model, the subject matter is broken down into entities (subjects) and relationships among those entities. All data is seen as describing some aspect of one of the entites or one of the relationships.

"Is-a" and "has-a" relationships are relevant at this level of abstraction. At this level, relationships are identified, but not implemented.

After creating a conceptual model of the database, but before creating the database itself, it's useful to go through a logical design phase, resulting in a logical model of the database. If the database is to be relational, it's useful to make the logical model a relational one. The relational model is the next level of abstraction.

This is where primary keys and foreign keys come in. These keys implement the relationships that were identified at the conceptual stage. This is how the relational model implements relationships. At this stage, you get involved with design issues like junction tables, table composition, and normalization.

In addition to the conceptual level and the logical level, there are the physical level and the script level. But these are outside the scope of your question.

The two kinds of relationships are features of the problem to be solved. foreign key references to primary keys are features of the proposed solution.

Upvotes: 1

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