[Year 12 IT Apps] Database relationships

[Mia Alexiou]

Hi all,

I'm an intruder here (I tech year 11 not year 12) but I thought I could
share my somewhat rusty knowledge of databases.

In response to Lisa's question the employee/department relationship is
one-to-many.   As for "true" one-to-one relationships I've never seen one in
action and so doubt they are common at a small database level.  That being
said, I can think of a couple of situations where they might be useful such
as splitting commonly needed data from that which is rarely needed, to
reduce the time it takes to run queries over a huge table.  Another reason
might be security, with highly sensitive data being stored in a separate
table.

A 0 or 1 to-many relationship is much more common and is what most people
(even in industry) usually mean when they talk about one-to-one
relationships.  These are explained nicely (under the one-to-one heading!)
at http://www.onlamp.com/pub/a/onlamp/2001/03/20/aboutSQL.html.

I hope this was helpful

Mia Alexiou
RMIT TAFE
Information Technology
Sessional Teacher


On Wed, Jul 23, 2008 at 6:10 PM, Lisa Imms <lisimms at copticvic.org> wrote:

> Hi Mark,
>
> Firstly, thanks for the resource :-)
>
> I hate to even contemplate disagreeing with you as my ITA class and I
> regard you as the ITA font-of-all-knowledge, but I will try this anyway.....
>
> Your example for one-to-one: "E.g. an employee's department data in one
> table matches a corresponding department in another table, allowing data
> relating to that department to be fetched."
>
> Wouldn't this be a one to many relationship? Even the line you have joining
> them has the 'many feathers' at the employees table? Although each employee
> can belong to only one department, each department can (and will) have
> multiple employees? So it is actually a one to many relationship?
>
> Am I correct in assuming that a well organised database would not have any
> one-to-one relationships? The two tables in a one-to-one relationship
> could/should just be merged into one table?
>
> Lisa Bairstow
> St Mary's Coptic Orthodox College
>
> ________________________________
>
> From: itapps-bounces at edulists.com.au on behalf of Mark Kelly
> Sent: Wed 7/23/2008 2:24 PM
> To: Year 12 IT Applications Teachers' Mailing List
> Subject: [Year 12 IT Apps] Database relationships
>
>
>
> Hi all.  It's been mighty quiet here for a while.  To perk things up,
> here's a little revision for database relationships (1-1, 1-many,
> many-many) that may or may not be useful.
>
> Cheers
> Mark
>
> --
> Mark Kelly
> Manager - Information Systems
> McKinnon Secondary College
> McKinnon Rd McKinnon 3204, Victoria, Australia
> Direct line / Voicemail: 8520 9085
> School Phone +613 8520 9000
> School Fax +613 95789253
> kel AT mckinnonsc.vic.edu.au
>
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