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I am having trouble grasping the idea of redundant data in DBMS. To quote my professor:

"For each unique PK value, there are many sets of data, then, those will be repeating group."

What does this mean? Can someone please illustrate?

This is what I understand:

Building table

++++++++++++++++++++++++++++++++
|Building#  | Room  | Cleaner# |
|------------------------------|
|B001       |  1    |   XYZ    |
|B002       |  1    |   XYZ    |
++++++++++++++++++++++++++++++++

Here, Cleaner is the PK of the cleaner table. So, does that mean that the repeating data would be room attribute? What if B001 was repeated in the table?

Chandrahas Aroori
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coder007
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  • Which do you want to know--What is repeating or redundant data in a DBMS, or what does the quote mean? They are two different questions. (Also the quote is poorly phrased). Please tell us what situation you are faced with that raises these question and which one you are asking. If you are asking about the quote, please tell us what context it appeared in. – philipxy Jun 11 '16 at 23:00
  • Re [repeating/redundant data](http://stackoverflow.com/a/32036030/3404097). Re [repeating groups aka multivalued attributes](http://stackoverflow.com/a/37483508/3404097). – philipxy Jun 12 '16 at 00:46
  • Ignoring the poorly phrased quote, read [this answer](http://stackoverflow.com/a/23202535/562459). – Mike Sherrill 'Cat Recall' Nov 18 '16 at 16:15
  • Possible duplicate of [Normalization: What does "repeating groups" mean?](https://stackoverflow.com/questions/23194292/normalization-what-does-repeating-groups-mean) – Chandrahas Aroori Mar 13 '18 at 04:54

1 Answers1

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Here we can not consider Room as redundant data, because one needs both the building and the room number to uniquely identify the tuple. If there is another table called Cleaner in which all the information of cleaners are stored, then there is no duplication.

What I mean is, this table has redundancy. If you look cosely the information about each cleaner is repeated. [Values of Name and Age are same for the same Cleaner. 

++++++++++++++++++++++++++++++++++++++++++++++++++++
|Building#  | Room  | Cleaner# |    Name  |  Age   |
|--------------------------------------------------|
|B001       |  1    |   0001   |   XYZ    |  27    |
|B002       |  1    |   0001   |   XYZ    |  27    |
|B002       |  2    |   0002   |   ABC    |  35    |
++++++++++++++++++++++++++++++++++++++++++++++++++++

To eliminate data redundancy we would split this into 2 different tables like so:

++++++++++++++++++++++++++++++++
|Building#  | Room  | Cleaner# |
|------------------------------|
|B001       |  1    |   0001   |
|B002       |  1    |   0002   |
|B002       |  2    |   0002   |
++++++++++++++++++++++++++++++++

++++++++++++++++++++++++++++++++
|Cleaner#   | Name  |   Age    |
|------------------------------|
|0001       |  XYZ  |   27     |
|0002       |  ABC  |   35     |
++++++++++++++++++++++++++++++++
Chandrahas Aroori
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