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2
votes
1answer
156 views

Configuration options for relational algebra evaluator “ra.jar”

I am participating in the Stanford Database Class, and there are some problem sets to be solved in "mathy" relational algebra syntax. The course instructors demand from students to use this ...
1
vote
1answer
30 views

What is the difference between a union and an “isa” disjoint relationship?

To me, it seems that a union and an "isa" disjoint relationship are the same. What is the difference between the two?
3
votes
0answers
51 views

Logical optimization of joins and selections

I am studying query optimization. I have read that it might be preferred to apply relational selections before a logical join operation. Is there a scenario where joining first and then applying ...
2
votes
0answers
265 views

Transform XPath map into XML document using relational data

Background Most modern databases have XML functions that can be used to extract data in an XML format. I want to avoid the task of manually calling XML functions to extract the data. This problem ...
1
vote
0answers
30 views

Keys of a relation

I got relation: R = { (ABCDEF) , (A B F -> C, B C -> F, F -> A, F -> B, D E -> E, E -> D)} keys of this relation are: {BCE}, {EF} But i dont get it... why? Why not only {EF}.. it got less ...
1
vote
0answers
98 views

Closure of attributes definition clarification

A databases book I am using states [1]: The closure of {A1, A2, ..., An} under the FD's in S is the set of attributes B such that every relation that satisfies all the FD's in set S also satisfies ...
0
votes
0answers
15 views

Correct way of representation in tuple relational calculus

Given the schema: Customer=(C-name,C-street,C-city) Borrower=(C-name,Loan-no) Depositor=(C-name,Account-no) and 3 relations more(not useful in this context),the tuple relational query for: "All ...
0
votes
0answers
32 views

Decomposition into BCNF where an attribute appears in multiple functional dependencies

I am trying to decompose my relation: R = (A, B, C, D, E) with Functional Dependencies F = {D -> BC, E -> B, A -> B, B -> D} The problem is that D appears in two dependencies and in ...