View Full Version : Home Work

11-24-2003, 07:07 AM
Hi Everyone,
Ok this is my home work.. but i am not gona ask you guys to do it..

It is for the database class... I have modeled a Entity Relation Diagram (ER Diagram) for a given problem for which a database has to be created... I want to get the model right before i proceed to the next step.. i have created a model which i think is ok.. but i would like you guys to have a look and recommend any modifications which might improve the model.. or a better way of doing it..

thanx in advance

11-24-2003, 07:43 AM
Well your double ended relationships seem wrong (and confusing)

For example

+============+ +============+
| | 1 + | |
| Site |-------------------| Grid Ref |
| | | |
+============+ +============+

A site is composed of many grid references
A grid reference belongs to one site

Depending on what your local conventions are on multiplicity, you might consider
{m,n} - between m and n
{m,} - at least m
{,n} - at most n
* - zero or more, same as {0,}
+ - one or more, same as {1,}

For me, I would remove Species_Ref_No from Site_record (a site record will contain many species, so it can't really be here), and remove SiteID from Site_Species_Record, and replace it with a Site_record (so you can easily get date and recorder).

So it would read
A Site has many Site Records
A Site Record has many Site Species Records

11-24-2003, 09:33 AM
thanks... for the valuable response.. i am working on that now.. I think a grid reference can belong to more than one site since sites overlap..

any way will post the new model.. hope to get some review again..