AQA COMP3 June 2012 Exam Thread

Edited it cause I wrote it the wrong way round haha. The above is <integer><operator><integer> so that obeys the expression

(I haven't exactly been on point because my head's been bombarded with a lot of info today D: )

You beat me to answering him I need some help too though.

A person who collects coins has decided to record the details of his collection in a
database. He has decided that the following details should be stored for each coin.
CoinID
Type (for example 10p)
Year
Condition (for example “very fine”)
Value
Date purchased
SupplierID
Supplier’s Name
Supplier’s Address
Supplier’s Telephone Number
Price paid

When the data is stored in a normalised database two relations, Coin and Supplier, are used. For each of these, complete the following Data Definition Language (DDL)
statements to create the tables, including all key fields. The two ddl headings for this are coin and supplier. I don't understand why SupplierID goes in the Coin table instead of CoinID being a Foreign key in the Supplier table.

And Backus Naur Form. I still don't understand this and we only glossed over it at college and the book isn't helping me.

<expression> ::= <integer> | <integer> <operator> <expression>
<integer> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
<operator> ::= + | - | * | /

I don't really understand the first lineof it means. Is it saying the operator has to come AFTER the integer?

FFS i am absolutely rubbish at entity relationships man! just cant get my head round the question properly 9/10 times i get them wrong -.- >:O

yeah man I totally feel you there. Good thing they're only a couple of marks

~

is it possible we get a question asking us to write relations in fully normalised form for an un-normalised relational database? i am not good with them and pretty unconfident since our teacher had no clue about 1NF, 2NF and 3NF :L

Fairly simple
1NF = No repeating attributes (fields)
2NF = No partial non key dependencies
3NF = No Non key dependencies


Normalisation improves the design of the database allowing searches to be quicker.

And Backus Naur Form. I still don't understand this and we only glossed over it at college and the book isn't helping me.

<expression> ::= <integer> | <integer> <operator> <expression>
<integer> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
<operator> ::= + | - | * | /

I don't really understand the first lineof it means. Is it saying the operator has to come AFTER the integer?

I might be wrong, but I remember my teacher telling us to say repeating groups instead of repeating attributes. Apparently, a mark scheme said reject repeating attributes.

hmm first i've heard of that, i've learnt it in Computing and ICT and it has always been attributes. couldnt of been a new mark scheme so was probably of the old spec. Double check but i've always said attributes and its been fine.

When do you use/ how do you know when to use two attributes as a single primary key to a relation? i cant figure out why you do it and how you know which attributes in a table/relation to pick for the primary key and its pissing me off now -.- can someone please explain?
many thanks

Yea I think it was from a very old mark scheme and he is ancient as well, just a minor thing :P

We usually create a ID field which is simply either a number or a short string. For example Teacher ID = 1, Supplier ID = KELLOG

We simply choose a field which is unique, so for example there is a chance that two teachers could have the same first name so we cant take that chance and make First_Name as the primary key. The same with Supplier ID we may get a situation where two companies share the same postcode (random but go with the flow)

Basically choose a field or create one that is unique

It depends if each coin can have many suppliers and if each supplier can supply many coins.

Case 1: A coin has 1 supplier and suppliers can supply many coins - Supplier ID in the Coin table
Case2: A coin can have many suppliers but each supplier can only supply 1 coin - CoinID in supplier table
Case 3: A coin can have many suppliers and suppliers can store more than 1 coin - You need a table in the middle or the Coin table and the Supplier Table which only holds the link between the two tables (as to break the many to many relationship).

I understand you, and i can probably use this example to wing it in the exam. But 'why' is it like that? Sorry to be a pain. Am I missing something? Is it really simple?

Not really looking forward to this exam, and I feel that abstraction/information hiding and intractable problems may also come up