Model (Database)

Model, Domain Model, Relational Model, Keys (Super, Candidate, Primary)

What is a Model ?

Model is an Object. Anything that exists in the word can be called or represented in RDBMS as a Model.

For example, Car, Person, Employee, Telephone, Tree, Country, etc.

You may compare Model with Noun in human language.

It can represent a People, Place, Thing and Idea.

What is a Domain Model ?

Domain means a context or a specific business boundary. A closed area which is connected with other areas.

You may call your entire business as a single domain OR each department as separate domains.

For example:

HR domain, Product domain, Sales domain, Customer relationship domain, Tax domain, Electrical domain, Electronics domain, Civil Engineering domain, etc.

Domain model represents business data and some behavior.

Domain can have sub-mains as well. (Core domain & Sub-domains)

Domain Model = Domain Data + Behavior

What is a Relational Model?

Two or more models which are related to each other.

For example: Employee works in a Department. An Organization has many Departments.

Understanding a Model = Understanding SET Theory

Have a look at the relationship between the two models Product & Customer.

Relation Mapping (Product/Customer/Mapping)

Relation is represented by keys and relation-name.

What are Keys?

Like in real word, each lock has its own key.

In RDBMS, key is an attribute used to uniquely identify a record in a table.

3 types of keys [Super Key, Candidate Key, Primary Key]

Super Key

One column or combination of two or more columns uniquely identifies a record in a table, then these sets of keys are called Super keys.

Example: consider the above Customer table

Super keys are [Id, (Id + Name), (Id + Type) + (Id + Name + Type)]

With the combination of (Id + Name) we can uniquely identify a record.

With the combination of (Id + Type) we can uniquely identify a record and so on.

Problem in super key is that it contains possible combination of all the columns in a table. Here in the Customer table we’ll be having 4 super keys.

Candidate Key

Minimal set of columns which can uniquely identify a record in the table.

For example:

  • Id is a Candidate key in customer table
  • Id is a candidate key in Product table
  • Name could also be a candidate key in Product table.

A candidate key can never be NULL or empty and should be unique.

There can be more than one candidate keys for a table.

A candidate key can be a combination of two or more columns of a table.

The problem with candidate key is that there could be multiple candidate keys in a table.

Primary Key

Primary key the most optimal candidate key. It is a simple key on a single column.

For example:

Id is the Primary key (Product, Customer, etc.) tables.

There can be only one Primary key in a table.




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