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Identify Cardinality

Posted on 2007-04-06
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Last Modified: 2012-08-14
Do anyone know how i can identify relationship verbs or cardinality marks in my ERD?

Reference_Food_Category
PK food_category_code
Food_category_descritpion


Reference_Animal_Breeds
PK animal_breed_code
Animal_breed_full_name
Animal_habitat
Countries_of_orgin
Animal_breed_other

Reference_Dietary_Requirements
PK animal_breed_code
PK food_item_id
FK Daily_requirement

Reference_Food_Items
PK food_item_id
PK food_category_code
Food_item_code
FK Food_item_description

Animal_Checkups
PK animal_id
PK checkup_date
Checkup_height
Checkup_weight
General_health

Animals_in_Zoo
PK animal_id
Animal_name
Gender
Birth_date
Arrival_date

Animal_Specific_Dietary_Req.
PK animal_id
PK food_item_id
FKDaily_requirements_description
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Question by:shoris
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8 Comments
 
LVL 42

Expert Comment

by:dqmq
ID: 18868230
You can't.  The Relationships window in Access is not really an ERD (Entity Relationship Diagram).  It shows tables and foreign key constraints, which is not quite the same thing.  A foreign key does not express cardinality, except for designating which table is the "many" side (if you can call that expressing cardinality).  Of course, the "many" side is quite evident in both the Relationship window and the query designer window in Access.

To depict cardinality, optionality, relationship names, role names and other logical constructs you need a different tool. Good tools are quite pricey, but for something economical, you might want to have a look at Visio professional.
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LVL 75
ID: 18868605
dqmq:

Cool post !  :-)

mx
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Author Comment

by:shoris
ID: 18869282
Ok.. that makes sense but how would i should in Visio the cardinality from the above tables? maybe i'm just confused of cardinality..
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LVL 57
ID: 18869347
<<maybe i'm just confused of cardinality>>

   Cardinality is the measure of how unique the data is or in other words, how many different members there are in a set.

  Typically you talk about cardinality in when working with indexes.  For example, if you have a yes/no column, it generally makes a poor choice for an index because there are only two possible values to the data.  Indexing doesn’t make sense because you’ll end up doing a table scan anyway.

    Usually, it’s expressed as a percentage.

HTH,
JimD
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Author Comment

by:shoris
ID: 18869352
Ok that makes sense.. thanks Jdettman, but in my tables above what would be cardinality?
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Author Comment

by:shoris
ID: 18869394
Based on my tables, can anyone understand the relationships?
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LVL 57

Accepted Solution

by:
Jim Dettman (Microsoft MVP/ EE MVE) earned 500 total points
ID: 18869634
<<Ok that makes sense.. thanks Jdettman, but in my tables above what would be cardinality?>>

  You can't answer that until the tables have data.  The question you need to answer is "How unique is the data in the column?"

   An autonumber column would have a high cardinality; every row would have a unique value
   A column with a customer name would be fairly unique (lot's of different names, but there could be quite a few Smiths)
   A yes/no column has a very low cardinality; there are only two possible values that would appear in the column.

<<Based on my tables, can anyone understand the relationships?>>

  Yes.

JimD.
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LVL 42

Expert Comment

by:dqmq
ID: 18871564
OK, there is some confusion here about different uses of the term "cardinality".  Since you mentioned ERD's and relationships, I gave the answer in that context. The realtionship cardinality is often depicted in an ERD and indicates how many entities may participate on each side of the relationship.  Examples are 1:1 (one-to-one), 1:M (one-to-many), 0:M, M:M and so forth.  That information is usually conveyed in the sybology of the particular diagramming technique.  

Row cardinality refers to the number of rows in a table,  but is NEVER shown in an ERD.  Column cardinality refers to the diversity of values in a column. While very important to index choice, and is not relevant to an ERD, either.
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