Tagged Questions

the study of relations (i.e., database tables) and relational operators (join, selection, projection, etc.) as a kind of algebraic structure.

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Relational Algebra - Finding statement

I got the following database schema and I am trying to formulate a few relational algebra terms, but I have still some issues with formulating them: For each flight with the aircraft capacity of at ...
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How do I represent an outer join in tuple relational calculus?

I know how to make an outer join in relational algebra but not in relational calculus. let's say I have R1(A, B, C) and R2(D, E) how can I make an outer join between R1 and R2 with the condition A = D ...
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Proof of RAP-derivation sequence for a set of functional dependencies in relational databases

I'm reading David Maier's outstanding but out-of-print book on relational databases (the book,"The Theory of Relational Databases", is available online from the author's website http://web.cecs.pdx....
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Count in relational algebra using Ω = { π, σ, ⋈, ⋉, β, x, ∪, ∩ , - }

schema(author)={name, newspaper} The table contains a list of authors. Each author works for a newspaper. So a single newspaper may employ multiple authors. How ...
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convert query to relational algebra

I have the following relational database schema: and I want to convert the following into relational algebra: List the names and phone numbers of the bidders who are not always affected by ...
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Expressing at least one in relational algebra

I began realtional algebra and I have some difficulties with the division which means for me "every". Yet every time I use it it seems to be playing the Russian roulette. Let be three databases: ...
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Relational query for universally quantified formula

My university is participating in the implementation of a library borrowing managing system at Richelieu National Library in France. I received the order to formulate the query: "find all users ...
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NL and NP compute different binary relations, so what?

Let the binary relation computed by a nondeterministic transducer be the relation between input strings and the possible output strings the transducer can produce (and accept) for the given input ...
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Combinations of elements with mutual relationships

I need to create the combination of 3 elements from a array of given n elements, however every of those 3 el. has to be in "relationship" with each other. Here is an example I can describe my issue ...
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Using cross-products with relational algebra

I have some questions about something that I'm working on in an assignment for my database class. Here is the schema for the questions I'm working on, so that you know what to refer to: ...
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A relational algebra extended to model the full DML (“CRUD”) domain

There are multiple references about the relational algebra for modeling queries (SELECT) but I have found very very little on the expanded algebra that would ...
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Algorithm to test dependency preservation

Is there an algorithm to test whether the decomposition of relation $R$ into relations $R_1,\dots,R_n$ is dependency-preserving? In more detail, I am given a relation $R(x_1,\dots,x_m)$ on $m$ ...
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Finding the Candidate Keys of a Relation Using the FD's

I have definitely checked out many different related posts. I have also done different sample problems from online sources as well from a similar problem. However, I am stuck on the problem below ...
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When should you use the existential and universal quantifiers for Relational Calculus?

Could someone explain to me WHEN do you use existential and universal quantifiers for Relational Calculus? Given this schema: Hotel(hotelNo, hotelName, city) ...
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Is existential quantifier redundant

I was reading Tuple Relational Calculus from a textbook. I came across this query: For every project located in 'Stafford', list the project number, the controlling department number, and the ...
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Relational Algebra - Mistake in the quiz?

I've just done a mini test and scored 9/10, however, from my understanding, I think this question is incorrect. Here is the only question which I don't understand how it can be correct: So I ...
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Use count() in relational algebra

Person(name, phone) Mike 456-789 Mike 123-456 John 230-785 $\dots \dots$ How to get those people who have more than one phone numbers with relational algebra? This sounds simple, but I am ...
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convert domain relational calculus with two same tuples to relational algebra

Given two relations $R(A, B)$ and $S(A, C)$. I can understand the early part is the projection of attribute $A$ from $S$, which could be written as $π_a(S)$, but later part is confusing and not ...
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In search for toy relations to test relational algebra

Does anybody know any toy database / relations that are particularly good and also complete to demonstrate and test relational algebra concepts? I'm hoping some database researchers should have ...
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What does superscript + mean in relational algebra?

What does superscript + mean in relational algebra? Suppose $A$ is a subset of attributes of all attributes in a relation $R$. What does $A^+$ mean?
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Finding representatives of equivalence classes

I have a table of pairs of objects, which defines an equivalence relation. How can I extract a single object from each of the equivalence classes using relational algebra? It can easily be done with ...
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Resolving many to many relationships?

I'm trying to separate a weird relationship. It goes like this: One waiter can serve more than one kind of drink and one kind of drink can be served by more than one waiter. One waiter can wear more ...
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Relational algebra statement for a query

For the following schema ...
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Equivalence relation Mathematical base In Computer science

I have this equivalence relation problem: Let $R$ and $S$ be two equivalence relations on the same set $A$. Define a new relation $U$ such that $U(x,y) \leftrightarrow [R(x,y) \text{ or } S(x,y)]$. ...
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Smallest set of features that would make relational algebra Turing complete

I'm thinking this should be just one or two things, since lambda calculus is so tiny and still Turing complete. Probably just recursion (something like "MY_QUERY(param) = select * from param UNION ...
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“At least one” clause in Relational Algebra

I'm fairly new to the syntax of relational algebra, and I'm having a hard time understanding how I could set a "at least one" clause. Example: I have: a table with books (listing the title, year ...
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relational algebra and SQL

Using the following relations: ...
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Computing number of block reads given relational algebra statement

So I'm just starting to learn about query processing and such in databases and I'm having some trouble. I don't really understand how to compute the minimum number of block reads given a relation and ...
110 views

Origin of theta-joins

I've taken an introductory database course where like many good database courses, they taught relational algebra, including $\theta$-joins. However, I've recently read E. F. Codd's 1970 paper, A ...
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What is OUTER UNION and why is it partially compatible

I am trying to understand how a OUTER UNION $∪^✳$ works, and why it is only partially compatible. I am aware this operation was created to take union of tuples ...
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Optimizing a join where each table has a selection

Consider the following query: ...