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Results tagged with relational-theory
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user 2047
On this site this tag applies to questions about Relational Model theory. The Relational Model for database management is an approach to managing data using a structure and language consistent with first-order predicate logic. In the Relational Model of a database, all data is represented in terms of tuples, grouped into relations. A database organized in terms of the Relational Model is a relational database.
0
votes
Accepted
Relation candidate keys
A superkey (and therefore a candidate key) that does not contain all attributes must contain at least an attribute set that is the left side of a functional dependency. Otherwise it would not be possi …
- 7,752
5
votes
Accepted
Are three equalities on a join operation legal in relational algebra?
The notation Producer=Actor=Director is wrong. Besides that, neither ΠActor(Film) nor ΠProducer(Produce) has an attribute Director.
The first projection has only an attribute Actor, the second projec …
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3
votes
Check if relation is in BCNF
"Armstrong's axioms are a set of axioms (or, more precisely, inference rules) used to infer all the functional dependencies on a relational database"(link).
R: ABCDE
F: C->AB, D->A, BE->CE, E->B
B …
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1
vote
Decomposing a relation into 3NF/BCNF
No, your decomposition is noteven in 2nd normal form.
{A -> B,C}
{E -> F}
{D -> I}
{A,D -> G}
{G -> H}
A, D and E cannot be found on the right side of a dependency, so they are members of every ke …
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5
votes
Why the term "relation(al)"?
Relational databases is based on the relational model of E.F.Codd. The relational algebra describes methods of how to query data.
A relation is simply a subset of the crossproduct of some sets (domain …
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5
votes
Accepted
Primary key in resulting relation from union operation in relational algebra
By definition the union of two relation contains no duplicates. So it contains at least one candidate key, the composition of all attributes. In your example (id, name) is a candidate key.
DBMS do no …
3
votes
Accepted
TRC: Select an ID if it appears on all measurements
I had to look up wikipedia because I did not know Tuple Relational Calculus. I did only skim over that article. There are differnces in notation as far as I can see.
But after activating my knowled …
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1
vote
Which tables to create according to relations
An ER diagram I deduced from your descritption. I am not sure if this is a valid interpretation of your description. ER diagrams and the transformation of ER diagrams to relations are discussed in t …
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3
votes
How to represent NULL in relational algebra?
There are no NULL values in relational algebra. Here you can use the antijoin operator ▷:
πROLLNO, NAME(STUDENT ▷ENROLLMENT)
that selects all records from STUDENT where the ROLLNO is not in ENRO …
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