Relational Algebra

Question

In relational algebra query , it always remove duplicates,

So, do we need to use cross product always, in place of join or union operation ? If not , then where exactly we use join or union operation ?

Comments

mam, i didn't get your question, can you elaborate more?

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CROSS PRODUCT got all attributes if r1 has m and r2 has n then total m*n.

see condition of union there must m=n so in union attributes less.

see join as join is select and cross product also removing some rows .

cross product acquire large space and almost everything

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@Shaik

In RA among for fetching some rows do u prefer cartesian product or join or union?

and why do u prefer??

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In RA among for fetching some rows do u prefer cartesian product or join or union?

it's depend upon the query.

But note that the output( not final ) from the usage of cartesian product is different than output of union.

 

If you are interested in Final output only, For getting duplicates we have to use Cartesian Product only !

( choice between join and cartesian product is  depend upon our wish and query )

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See I am asking only RA

right?

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For example See here why B) or C) give incorrect result?

https://gateoverflow.in/920/gate2003-30

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mam, give with example which convey your question clearly !

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given..

Even I read in Navathe that Cartesian product and Join produce same result in RA

So, getting stuck here in concept of book and concept of GATE question

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mam , in join we have to see that in two relations one attribute is common or not .

but no such case here in cartesign product we just multiply  all other attributes with each other

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In that gate question "where p " is a condition chacking

rt?

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@srestha, mam

In that gate question "where p " is a condition checking

yes.

 

actually, Option C need not to be true.

i mean, For cartesian product we don't check union compatability, but for performing union operation those tables must be union compatiable.

 

Coming to option B, it is natural join. ( may not be natural but it should be join )

natural join means, the common attribute equal values only present in the result.

r1.B =r2.B ===> the rows like 1,1 or 2,2 or 3,3

But by the cartesian product, it need not to be r1.B =r2.B ===> the rows like 1,2 or 2,1 are also present.

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ur logic contradicts here

https://gateoverflow.in/19838/gate1997-76-a

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what is the contradiction ?

 

Note that join = cartesian product + selecting condition.

Answer 1

join is actually a combination of ( cartesian product, select ).

join opration pair two tuples form  different relations, if and only if a given join condition is satisfied.

cartesian product alone is meaning less .so we use it with select as a join. it is need not be union compatible.