A “part-key”

database-theoryprimary-keyrelational-theoryterminology

I'm a beginner and student with regards to database design. The tutor recently discussed “part-keys”. I am struggling to find any information on what a part-key is, and I suspect it probably goes by another name.

The example we were given in class was this:

ACTIVITY(Activity(PK), Fee)
STUDENT_ACTIVITY(StudentID(PK), Activity(PK))
Is the table showing part-key dependency?

Yes. Since the second determinant (Activity) is part of the primary key (StudentID, Activity).

Is anyone able to give me any further information?

Best Answer

This example does not contain a partial dependency to me.

A partial dependency would be if a table column functionally depends on part of the table's primary key, like in this denormalized table:

CREATE TABLE student_activity (
   studentid bigint NOT NULL,
   activity character varying(30) NOT NULL,
   fee numeric(10,2) NOT NULL,
   PRIMARY KEY (studentid, activity)
);

where fee depends on activity.

The solution would be to split this table in two parts like in your question.

Related Solutions

What do you call something that’s not the primary key

User terms surrogate key and natural key for primary key as a variant.

Surrogate Key:

Surrogate keys are keys that have no “business” meaning and are solely used to identify a record in the table. Such keys are either database generated (example: Identity in SQL Server, Sequence in Oracle, Sequence/Identity in DB2 UDB etc.) or system generated values (like generated via a table in the schema).

Natural Key:

Keys are natural if the attribute it represents is used for identification independently of the database schema. What this basically means is that the keys are natural if people use them example: Invoice-Numbers, Tax-Ids, SSN etc.

Surrogate Keys vs Natural Keys for Primary Key

What scope(s) is “hasMany” a part of

In any case, when one thing hasMany of another how/where does it show up in application and database logic?

The only way I can think to answer this question is with another example of hasMany that may or may not answer your question.

an example of a hasMany using posts and comments

err, except I should have called the table "comment" but I think the point stands.

I don't know that this really helps you understand anymore than what you already had.

So I'll also share a query that might demonstrate how this works:

declare @var char(5)
set @var = 'query'

select
   c.id
  ,c.text
  ,c.author
from comment c
inner join post p
   on p.id = c.parentid

which we expect to return many rows, because one post generally has many comments (in theory, of course not all posts will have comments)

Update:

So here's the thing about relations: They are EASY to model via diagram (see the very first tool I pulled out to answer this question) but they are HARD to diagram via text. So instead of trying to define whether we mean 1:M or 0:N, we just write it in "near English" and say what we intend. Here's some alternatives:

one to one           hasOne               post hasOne author (not true for CW ;])
one to many          hasMany              The one in question
many to one          belongsTo            parent child, comments belongTo post
many to many         hasAndBelongsToMany  This describes a complex relationship
many to many to many hasManyAndBelongsToMany

As you see they can get quite complicated. This would ... once again! ... indicate a need to refactor your business logic into simpler and smaller chunks.

These can also be called "association types" and may be indicated by verbage like "WrittenBy" (a many to one) or "IsOwnedBy" (one to one? - context is key)

Before I walk away from the "hasMany" concept for the last time (well this is an update, more will likely be written) I want to mention that while it's useful for "plain English descriptions", it's also used in ActiveRecord styled installations. Take, for instance, Ruby. http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html The rest of this post (so far) is taken from that page:

class Project < ActiveRecord::Base
  belongs_to              :portfolio
  has_one                 :project_manager
  has_many                :milestones
  has_and_belongs_to_many :categories
end

This allows the ActiveRecord ORM to create the data model and to maintain the relationships as specified by the business logic within the program. But I don't write Ruby or Rails so that's as far as I can tell you, I just know that it exists there.

Best Answer

Related Solutions

What do you call something that’s not the primary key

What scope(s) is “hasMany” a part of

Update:

Related Question