AQA COMP3 June 2012 Exam Thread

x Mod n is the remainder of x when divided by n.

For example 12345 / 6 = 2057 rem 3, therefore 12345 MOD 6 or 12345 % 6 is 3.

can two relations have the same primary key?

Yes.

i thought that a primary key in one relation can only be a foreign key in another...no?

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

Highly unlikely. They're more likely to give you a picture of a database table, and say why it isn't fully normalised. They might ask you to then give a defintion of 1nf, 2nf etc.

Could I get some help with this binary tree question? I'm not really good at these.

Could I get some help with this binary tree question? I'm not really good at these.

this is the answer i got. I think its right...hope it helps! :P

The first item is the root (Manchester)
The second item is Bradford, this is alphabetically before Manchester so you place it in the left subtree.
The third item is Halifax, this is also less than manchester so you place it in the left subtree, but Bradford already exists there, Halifax is > Bradford so you place it in the right subtree of Bradford.
The Fourth Item is Hull, Hull is less than Manchester, Greater than Bradford and less than Halifax so it goes in the left subtree of Halifax and so on until you come to the end of the list.

The challenge is not usually creating the binary but creating the binary tree so that is balanced. (What happens if you construct a Binary tree from an ordered list?)

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?

Yeah, think of it as the operator is followed by the integer.

But then i get confused because 4 * 9 'follows this expression' and that has an operator before the integer 9