Difference between Logical Data Modelling & ERD

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Difference between Logical Data Modelling & ERD

Post  sriramhariharan on Sat Jan 15, 2011 11:38 pm

What is difference between Logical data Modelling (LDM) & ER ?

Whenever we ask LDM , the customer gives ER diagram.


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Re: Difference between Logical Data Modelling & ERD

Post  ngalemmo on Sun Jan 16, 2011 2:08 am

The difference is between a logical model and a physical model. Both are presented as ER diagrams.

A logical model is slightly more abstract than a physical model. A physical model represents exactly what is implemented in a database... table and column names are as they exist in the schema, you define indexes, specify column datatypes and sizes, contraints, tablespaces, partitions and all the other requirements to actually define the database. A logical model shows entities, attributes, keys and relationships. Names are usually more free-form and descriptive.
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Re: Difference between Logical Data Modelling & ERD

Post  BoxesAndLines on Sun Jan 16, 2011 12:15 pm

When you ask for a LDM, what are you expecting?
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Re: Difference between Logical Data Modelling & ERD

Post  Jeff Smith on Mon Jan 17, 2011 6:48 pm

Don't "conceptual models" sometimes get confused with "Logical Models".

A conceptual model might have a fact table with the dimension tables but without any column names. Some of the dimensions might later be split into 2 or combined. It's almost a graphic presentation of the BUS Architecture.

The logical model might have the column names in lay terms - "Group Identifier" instead of group_id. There may not be difference between a view and a table.

And the Physical Model is what the database physically looks like.

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Re: Difference between Logical Data Modelling & ERD

Post  ngalemmo on Tue Jan 18, 2011 3:03 pm

Don't "conceptual models" sometimes get confused with "Logical Models".

Good point. Frankly, I'm confused most of the time. Maybe there needs to be a model model.
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Re: Difference between Logical Data Modelling & ERD

Post  BoxesAndLines on Tue Jan 18, 2011 5:19 pm

To me, there is no logical dimensional model. Conceptual and logical models are normalized data structures used to define data dependencies. Unfortunately, there is no consensus on what goes into a conceptual model other than it is less than the logical data model.
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Re: Difference between Logical Data Modelling & ERD

Post  Jeff Smith on Wed Jan 19, 2011 1:14 pm

How's this...

A Conceptual Model is what you on a scrap piece of paper.

A Logical Model is what you do in Excel.

And the Physical Model is what you get when you use Visio to reverse engineer the database.

Actually, a variation on the conceptual model comes in handy when setting up the reporting tools.

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