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Posted on 2014-12-21
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I am reading about functional dependency where it says:

"In some cases, functionally dependent sets are irreducible if . . .The left-hand set of functional dependency cannot be reduced, since this may change the entire content of the set"

at http://www.techopedia.com/definition/19504/functional-dependency (see: Techopedia explains Functional Dependency)

Question: Could you please make up some data in a table to explain left-hand set by an example?
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Question by:Mike Eghtebas
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by:Dave Baldwin
ID: 40511973
They mention an example with a SSAN in that article.  That was a pretty good example.  If the left-hand set is a unique key, then it is irreducible because changing it would point to a different right-hand set of data.  Same as an auto-increment id key in a database table.
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by:Mike Eghtebas
ID: 40511982
Hi Dave.

What left-hand set stands for.

"example with a SSAN in that article" do you mean the example in the video? If so, I am sure you have understood it. But I didn't and this is why I posted this question asking for another example. Possibily with a diiferent table name and field names.

Regards,

Mike
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by:Dave Baldwin
ID: 40511989
What video?  There's no video in the link you posted.
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by:Mike Eghtebas
ID: 40511995
Sorry, slide. I have another question related to a video. I mistakenly wrote video.
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by:Dave Baldwin
ID: 40512083
You must be over-thinking this.  For a given table, all it takes is an 'id' column like the SSAN and a 'name' column.  As mentioned in the article, an SSAN (Social Security Account Number) can't be 'reduced' because they are supposed to be unique and identify one and only one person.  Where as 'John Smith' can be any of thousands of people.  The 'name' column can be reduced to a Foreign Key that point to a table of names where each 'name' is listed once and only once.  Every 'John Smith' would have the same Foreign Key in the first table.

In practice, this is rarely done because most of us don't have tables that have enough duplication of names like 'John Smith' for it to be worth it.
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by:Mike Eghtebas
ID: 40512148
You must be over-thinking this.

I have to either fake it and say I understand (this seems what you are suggesting) or ask about it (which you are not willing) to give me an example like.

Table1
===========================
field1          field2                field3
---------         -----------            -------------
x                    x                          x
x                    x                          x
x                    x                          x
x                    x                          x

Xs are data you insert to demo the concept in question.

I need something like this. Possibly because I am visual person.

Can you help me with this type of explanation?

Mike
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by:Dave Baldwin
ID: 40512198
No, I'm saying that you are making this seem harder than it is.  If you can draw that example, why can't you draw one with 'id' using SSAN and 'name' using John Smith and his friends?  'id' being the 'left-hand' and 'name' being the 'right-hand'.
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by:Mike Eghtebas
ID: 40512459
If I were to explain something to someone then I start the table the way I want to make the point and illustrate the question the way I want to. Here, I suppose (only if you are willing) need to start the table the way you want it. It should support whatever is in your mind and you are willing to talk about it. If this is not how you want to help, then please ignore this post.

Regards,

Mike
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by:Dave Baldwin
ID: 40512478
Is this so hard?

Table1
==================
id                     name    
---------            -----------
111-22-3344   John R. Smith
113-23-3345   John T. Smith
211-22-3346   Fred Smith
311-32-6344   Marta Smith
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by:Mike Eghtebas
ID: 40512502
Thanks,

Question: What is  left-hand set in this table you have made? This was my original question.
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Dave Baldwin earned 500 total points
ID: 40512508
And I already told you it is the 'id' column with the SSANs.  And it was in the article that you posted a link to.
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Author Closing Comment

by:Mike Eghtebas
ID: 40512514
Thank you. What was a bit confusing to me was the use of SSAN (as opposed to SSN or SS# I am used to).

I didn't know what you are referring to. And also, you may have implied ID is left-hand set but never said it explicitly. Yes, they probably have mentioned in the slides and I couldn't get it hence the question here.

I am glad it is over. My apology for not being able to handle this post more smoothly.

In all honesty, I am not quite satisfied with the answer because there could have been a bit more explanation as to why a single field is referred to as set. Possibly some contrasts etc. and expanding it for a better comprehension.

Regards,

Mike
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by:Dave Baldwin
ID: 40512533
You're welcome.  It is referred to as a set because, especially in a theoretical discusson, it does not have to be a single item.  The left-hand set part probably refers to the fact that keys are almost always on the left-hand side (in English at least).  Also mathematical... x=y  x on the left hand is unique.  y can be any expression that results in x.
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by:Mike Eghtebas
ID: 40512536
Thanks.
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