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Thread: ER-diagram, min / max notation
May 4, 2003, 05:38 #1
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ER-diagram, min / max notation
recently the topic min / max notation was introduced in one of my courses as a more exact way of expressing the cardinality between entities.
I have one question regarding the placing of the cardinality between entities:
"A house has several rooms" that means there is a 1:n relationship between the entity-types "houses" and "rooms". Where do you usually put the "n" and where the "1".
I would place the "n" at "rooms" and "1" at "houses". I have also seen the other way.
Where are the cardinalities usually placed?
My next question is in regards to the min / max notation. I think, I have understood how it works. What confuses me though is that the notation is completely inverse to the 1:n.
I have attached an image with an example of what I mean.
Thanks in advance,
May 27, 2003, 01:43 #2
well, a proper ER Diagram, or ER Model should follow SSADM Methodologies.
So, thrat being my presumption, is a 1 to many, 1 to 1.
As in, a house has many rooms and a room has one house.
the diamond shape shouln'r be there, there "have" should be documented just below or above the line.
as for your entity names (house and rooms) they should be singular not plural, NEVER use plural names.
House and Room.
The relationship naming convention, done properly should be one of three notes, one for each type of relationship.
1 to 1: (1:1)
1 to many: (1:n)
many to many: (n:n)
Right, the 1's and n's should be placed on the side of the corresponding relationship. If that makes sense.
You do not need two relationship notes at either side.
I've attached an image of how it should look.
Also, if you want to be proper, you have dependancies, which are a small circle at either end, a filled and not filled one, filled means MUST HAVE (compulsary) and not filed means CAN HAVE (not compulsary)