Re: Unfathomable ERD Relationships

jbotteron_at_pps.k12.or.us (Jack Botteron) wrote:

>In order to learn Oracle's technique, I created two entities in their
>ER Diagrammer: A "purchase order" entity on the left, and a "line
>item" entity on the right. Then I created a relationship signifying
>that one PO MAY contain none or many line items, and a line item MUST
>belong to only one PO. The diagrammer drew a line with a crow's foot
>(cf) as follows: cf with a solid line on the left end, and a
>dotted line on the right end.

When drawing a relationship, the ERD diagrammer always draws the "Many" crows-foot end first. So you need to draw the relationship from your Line Item to your PO.

This will screw you up if you then go and add description text to the relation ends assuming that it had been drawn the other way :-)

>When I created the report "Entity Relationship Diagram Details"
>the report seemed to accurately follow their conventions, but when I
>created the report "Relationships", the report displayed just the
>opposite cardinality and optionality.
Which is what I think happened to you here...!

>Also, the edit relationship dialog
>box is as confusing as the help screen associated with it. This
100% agreed! BTW, don't bother putting text into the "Text" tab of the relationship dialog box - it doesn't save it. (You can put it in using the RON though)

Try the help from each of the relationship buttons though (Click on the ? and then click on one of the relationship buttons). It's much more useful!

>Who in their right mind would put FROM cardinality on opposite ends
>of the FROM optionality? And then in their helps only talk about
It makes sense if you read from "left to right" as it were. Eg.

        Purchase Order - - - - - -------< Line Item

Would be "A purchase order MAY have ONE OR MORE Line Items" Conversely, "A Line Item MUST have ONE AND ONLY ONE Purchase Order"

So your optionality comes from the near end of the relationship you are reading, and the cardinality comes from the far end. IMHO, it makes reading a relationship in english very easy.

>How can I draw diagrams that will be readable by users, when
>Oracle and I cannot fathom them? Is there a way out of this?
The users of the system I'm designing have never come across ERD's before, and are having no trouble!

Best of luck!

--
Simon Holt --  System Designer/DBA
** My opinions are not nescessarily those of my employers **
Britannia Zinc Ltd.  (+44) 0117 982 3646