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

@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.