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1.In case of lossy decomposition,when tables are merged then will newly created table contain either spurious tuples(extra tuples) or some tuples are removed from original table?

2.How the data is lost when newly created table contains some spurious table? According to me data should not be loss as it contains spurious tuples( extra info).

P.s plz correct me if im
in Databases by (9 points) | 19 views

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Adding extra wrong information is definitely gonna lossy  It'll cost you in different way in different scenarios


Here's a simple example

lets assume a very simple database of simple library..

now you have a table where you store name of a person and book issued to him.,

lets say at some instance this table exactly looks like this..

Name of person name or say a unique no of books issued
p1 B1, B2
p2 B3

now say brake this table into two, one with person, and other with name of books ( This is totally a hypothetical example, I know you won't be doing this in general, this is just to give you a hint or some point of view)


our new tables are


Name of person


Name of book


merge new tables by taking cartesian product

person x Books

Name of Person Name of Book
p1 B1
p1 B2
p1 B3
p2 B1
p2 B2
p2 B3

for simplicity Group this by Name of person

Name of Person Name of Book
p1 B1, B2, B3
p2 B1, B2, B3


Now, this new table has all information we had earlier,

p1 got B1, B2

and p2 got B3

additional to this we have extra false information..


Now imagine what happens, p1 walks in with B1, B2 hand both of them to you.. and say i wanna return them.. and want a clearance(no due) slip to close my account, you say okay, let me return these and check your information..

your table still show that p1 has B3 as well, you'll say.. oh you have one more book to return, first you have to give it back to get a no due slip..

but p1 is sure about it that he doesn't have any other book..

this is a problem isn't it?

although we still have our actual information, but because of some extra false information, our original information isn't useful anymore..

it is giving us some faulty info. So actually we lost some info instead of gaining any.


One more example is this say you have a banking database..

where you have normal bank account( say saving/current) linked with a loan account for all those customers who have taken the loans..


now say you performed decomposition on this table, and when you join them, you have all the previous information..

additionally you have linked one more bank account with a loan account you had previously..

now this is a real problem no one wanna go.. you don't want this kind of things to happen..


Although it seems like you have some extra rows or columns in your table, but in actual you losses the correctness and usefulness of actual information you had earlier. (this is how i imagined it might be)


These both example aren't practical, but when decomposition where after joining you add some faulty information, stuffs like this could happen, and you can mess up thing really bad such that, you loose significance of actual information as well.


Hope this till now clear some picture for you.. you could come up with hundreds of examples like this, where you add extra points, which will eventually lead to  lose the real information.

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