Databases: Primary keys

Databases are very powerful tools that allow us to search and sort data at incredible speeds. An essential feature of a database is making each record unique. To make them unique we using Primary Keys.

Primary Key

Primary key - A primary key is a field in a table that contains unique data.

Let's look at an example in the Criminal Table:

Table: Criminal

Attributes:

NI Number: String

Name: String

Date of Birth: Date

Number of scars: Integer

Home town: String

Which of these data items are unique?

Attribute	Unique	Reason
Home town	No	you might have several criminals living in the same town
Number of scars	No	you might have two criminals with the same number of scars
Date of Birth	No	you might have two criminals born on the same day, or twins who are criminals
Name	No	you might have two criminals with the same name. e.g. John Smith and John Smith
NI Number	Yes	this is unique for each person

There is a short cut to writing out database table designs, where the underlined attribute is the primary key. Note that primary keys are normally written first.

TableName(PrimaryKey, Attribute, Attribute, Attribute, Attribute)
Criminal(NI Number, Name, Date of Birth, Number of scars, Home town)

Questions

Exercise: Primary Keys Spot the primary key in the following table attributes, and write out the table design: Table: Car Registration number: string Colour: string Number of doors: integer Convertable: boolean Answer: Car(Registration number, Colour, Number of doors, Convertible) Spot the primary key in the following table attributes: Table: Student record Name: string Unique Learner Number: integer Date of Birth: date Mobile number: integer Answer: Student(Unique Learner Number, Name, Date of Birth, Mobile number NOT mobile number as they might share a phone with someone else.

Composite key

Composite key - Collection of attributes uniquely identify a tuple rather than just one

However, things may not always be so clearly cut. Take this a table of football players for example:

Table:Player

Name: string

Position: string

Number: integer

injured: boolean

Team: String

Which of these would be the primary key?

Attribute	Unique	Reason
Name	No	you might have several players with the same name
Position	No	you might have two goalies
Number	No	you might be storing the details of multiple teams, in which case each number has several players from different teams
Injured	No	several people might be injured at the same time
Team	No	several players can play for the same team

So what do we do? Before we make our database tables it's best to go through this process and we come to realise that we need to introduce another attribute that is unique for each player. We might even invent one, a playerID:

Player(PlayerID, Name, Position, Number, Injured, Team)

Another way of solving these problems is to try and find a combination of attributes that together are unique. This is called a composite key. Take a look at this example for houses:

Table: House

Number: integer

Road: string

Colour: string

Post code: string

Attribute	Unique	Reason
Number	No	you might have a 61 on London Rd and a 61 on Manchester Rd
Road	No	you might have multiple houses on the same road
Colour	No	more than one house might be green
post code	No	multiple houses might have the same post code

We could make a primary key up, but if you look carefully, we can use a combination of attributes. The house number and the road name combined seem to be unique. You can't have two 45 Belmont Close can you?.

House(Number, Road, Colour, Post code)

Does that sound ok? What about if we were storing data on all the towns in the country and there was a 5 London Road in Manchester and a 5 London Road in Winchester. This would mean that the combination was not unique. We might try using the house number and post code instead, and this combination is always unqiue, this is our composite key:

House(Number, Road, Colour, Post code)

Questions

Exercise: Composite keys Where applicable rewrite the table description with composite keys, primary keys, or add a primary key for the following tables: Receipt(CustomerID, DateTime, Total, StaffID) Answer: Receipt(CustomerID, DateTime, Total, StaffID) Match(TeamA, TeamB, Date, TeamAScore, TeamBScore, RefID) Answer: Match(TeamA, TeamB, Date, TeamAScore, TeamBScore, RefID) Phone(ModelID, Colour, Weight, Internet) Answer: Phone(ModelID, Colour, Weight, Internet) You might argue that if this was for a particular instance of a phone instead of for a model of phone you'd need to introduce a unique value through serial number or the like Cat(Colour, Weight, NumberofLegs, Name) Answer: There aren't enough fields here for us to find a unique one or combination. We could have two ginger cats weighing 1 kg with three legs called Phil. We therefore need to introduce a new unique value, CatID Cat(CatID, Colour, Weight, NumberofLegs, Name)