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# Questions tagged [relational-algebra]

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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### 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 $m$ ...
5k views

### 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 ...
3k views

### 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) ...
99 views

### 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 ...
113 views

### 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 joined ...
29k views

### 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 ...
776 views

### 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 ...
25 views

### 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 ...
102 views

### 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?
35 views

### 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 ...
2k views

### 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 ...
113 views

### Relational algebra statement for a query

For the following schema ...
493 views

### 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 $U(x,y) \leftrightarrow [R(x,y) \text{ or } S(x,y)]$. ...
191 views

### 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 ...
7k views

### “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 ...
157 views

### relational algebra and SQL

Using the following relations: ...
178 views

### 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 ...
121 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 ...
983 views

### 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 ...
120 views

### Optimizing a join where each table has a selection

Consider the following query: ...