# A-level Computing/AQA/Problem Solving, Programming, Operating Systems, Databases and Networking/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

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.

### 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

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)