tuple relation

Question

consider the relation r1(P,Q,R) and r2(R,S,T) with primary key P and R respectively. r1 contain 2000 tuples and r2 2500 tuples. max size of natural joint of r1 and r2..? plz explain..

Comments

The max size will be Min $(2000, 2500) = 2000$ tuples.

Here is the reference. 

Answer 1

See here R is common in both the relation .Also R is key in the second relation .In second relation R will have 2500 distinct value.

Also Relation r1 has 2000 tuple .So here every tuple in relation r1 will map with the relation r2 .So it will give result 2000 tuple.

So Maximum size of R1⋈  R2 =2000

Also Minimum tuple in R1⋈  R2 =2000

Comments

2000 tuples of r1 will map to r2.

500 are distinct tuples...ryt...??

why the max size will be 2000.. :'(

can u plzzzzzzzzzzzz elaborate

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why 500 relation r2 has key R so 2500 different value for attribute R.rt?

Now r1 has also attribute R which is common in both the relation but r1 has 2000 tuple only rt.

So all the common attribute will result in natural join ok.

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that means whatever the common tuples are the... suppose it would have been 2300 for r1..

then max size of natural joint would be 2300...ryt..?

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yes Here off course 2300 tuple.

I would suggest you take an example with less number of tuple .Then every thing will be clear.

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@manojk see " http://quiz.geeksforgeeks.org/gate-gate-cs-2004-question-14/ "  

here..it is clearly  shown that 5 entries are there in relation of natural joint of student and enrol..then how cm max is 8..??

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He has taken 5 tuple as an example to clear the idea .But here are 8 tuple in Enroll relation.

So all 8 tuple will result in natural join.

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ohhh..yahhhhhhh.... how stupid am i..:P

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and min and max will be same ...??

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Yes but with restriction if there is no foreign key then min tuple  will be 0.

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ya ..ok...

If 2 relation has a same primary key.. ( http://quiz.geeksforgeeks.org/gate-gate-cs-2004-question-14/ )

then it will not consider as foreign key...?

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yes see this too

https://gateoverflow.in/1011/gate2004_14

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@dhairya still any confusion ?

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in that link which u have provided...same as i have given u... but answers are different...

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in ur link why " Enroll table rollno is FK " why not they have taken student's rollno as foreign key..??

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follow gate overflow link.

min will 0 when it is explicitly specified in question that there is no foreign key constraint  .

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in ur link why " Enroll table rollno is FK " why not they have taken student's rollno as foreign key..??

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@ManojK ... if your assumed foreign key elements( R of r1) all are different from the elements of table r2..then what would be min ??

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min would be 0... ryt @manojk...??

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@vijay foreign key elements( R of r1) all are different from the elements of table r2 ?

then what is foreign key meant for ?

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No min will be 8.

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ohh..ya..al the tupls of foreign key will be there in other relation which it is refering to.. ryt..?

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@dairya i already told you it will return 0 when there is no foreign key .and there will be no common attribute in the relation.

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I should provide one line for answering this question.Then only it will be clear .Instead of up voting you down voting the answer.Ok No problem.If you have still any doubt you can ask.

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thanxxxx manojk i got it now....:)

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thank you .