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Understanding the joins in BMM 11g
Hi All,
I am curious to know why sometimes the Fact table appears on the left and Dim on right and vice versa. The cardinality reflects correctly under the table, but this confuses me whether it should be a left outer or right outer join. Please find the attached screenshot for reference:
Thank you.
Sia
Answers
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Hi,
The fact table is displayed depending on how you created the join: when you create the join you click on a table and then click on the second one, the join has an arrow. That arrow is the one defining which table is on the left or the right.
So it's your model, depending the joins you create (and their direction).
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Normally logical joins should go outwards from the fact table. So fact -> dimension. Your "issue" just means that your logical joins were created randomly.
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To expand; -
to resolve go to the PHYSICAL layer and look at the physical joins created there.
If any are NOT fact to dim (Fact -> Dim) then they are incorrect, and need recreating.
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@BeginnersMind You know that logical joins can exist totally independent of the physical layer, right?
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Yes, you are totally correct!
I did "know", but I had forgotten through long habit...
I usually model physical first and then logical.
My only action on the logical is typically to create the logical join fact to dimension, and on rarer occasions overrule the default on the join created.
As such I fear I had forgotten the "why" of the drag fact to dim and that it does not have to reflect the join "direction" of the underlying tables.
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Reflecting the physical world is only something that's of importance in the initial drag&drop to the BMM layer. But my point was also geared towards non-conformed dimensionality joins which only ever exist as a logical relationship at all.
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