Questions tagged [relational-theory]

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.

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Why big data analytics over peta-byte datasets leads to NoSQL not RDBMS [duplicate]

Computational and storage requirements of applications such as for big data analytics. business intelligence and social networking over peta-byte datasets have pushed SQL-like centralized datasets to ...
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1answer
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Immutable field/cell in Postgres

Is there a way to create an immutable field/cell in a Postgres record? AKA, only allow the field to be written once, perhaps just on insert? Use case: I have a "uuid" field for a record, and ...
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1answer
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Help designing a Table(s) with a single message to multiple placeholder values, with new placeholders added occasionally [closed]

The basic situation is I have a single 'message' table, and currently our design for this is a single row, with all placeholde values stored against the row at generation, so they are available for ...
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3answers
218 views

Why Cassandra is not considered a relational database?

Read this answer Relational databases are based on the relational model, an intuitive, straightforward way of representing data in tables. In a relational database, each row in the table is a record ...
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2answers
132 views

database management system vs database vs tables vs file [duplicate]

I have been trying to understand the way a database system works. Digging through the internet, I have come to following picture. Database system is required because we need a way to store data, ...
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27 views

“Tuples in a weak entity set are partitioned according to their relationship with tuples in a strong entity” - meaning?

http://web.cs.ucla.edu/classes/fall04/cs143/solutions/chap2.pdf In the above link, there is the following statement - "Tuples in a weak entity set are partitioned according to their relationship ...
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1answer
80 views

Incorrect relationships formed in EER diagram by reverse engineering the database script

I have a database with 6 tables. I tried to view the relationships between the tables by using EER diagram. I observed that all the one-to-one relationships were converted into one-to-many ...
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1answer
41 views

Data Schema For Stock Control / Multi Source Inventory

I'm working on a project that involves stock control with multiple stock sources and sales channels. The overall hierarchy I've got so far looks like this; Sales Channels <---- Allocated Stock ...
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2answers
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One and only one relationship - ER diagram to relational schema

I'm taking this term a DB course as part of my CS studies. This week we began studying the subject of ER diagrams, which I find a bit confusing. I have some uncertainty about the picture above (Note: ...
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2answers
27 views

Multiple Primary-Foreign Key connections between Tables Redundant?

I have two tables, Reviews and Critic I'm pretty new to RDBMS and I was wondering if the connection between Review.rID and Critic.Review is redundant? My reasoning is that a single critic can have ...
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1answer
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In what cases is using ClickHouseDb and the like a neccessity?

An open source for website analytics - https://github.com/plausible/analytics They use Postgresql and ClickHouseDb. When it comes to web analytics, there're tons of events going that need to be ...
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4answers
120 views

How to store a (record which holds a) reference to any other column (attribute) in another table (relation)

TL;DR: If the database schema should hold all the business logic, how is it be possible to specify that an attribute type is a reference to a specific attribute, instead of a specific record (as is ...
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1answer
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Is file system in DBMS same as file handling in C programming language?

We know that file system in not good for storing data , thus relations(tables) are used in DBMS. When we talk about files in database management system (DBMS), are these same/similar to that in C ...
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17 views

How to add an additional table over two one to many relations tables?

I have two tables TableA and TableB which have a relation of one to many Later I am told that there must be an additional table that links the initial tables with additional information. To carry ...
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23 views

Can a table be un-normalizable or am I misunderstanding the theory

I was considering normalizing a flat table (SQLite if its relevant) I'd been working with in a small personal project and it just didn't seem to make sense. Consider an example table like below. Each ...
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2answers
69 views

RDBMS: Proper way to store data - comma separated variables or in different fields or tables?

Fundamental question that I cannot seem to find an answer on. I have a database that stores line items from receipts along with the username and receipt number. Right now the lineItem column is just ...
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1answer
17 views

How can I calculate the cost of the query, if there is a `between` condition?

How can I calculate the cost of the query, if there is a between condition? Select * From A Where A.id between 10 and 50 If Index not exists and there is only one condition: Search in B+ tree to ...
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2answers
174 views

Incorrect result when using 'case when count()=0' & 'where'

I would very appreciate if someone could help me out with this query. I can't seem to get the correct result when using case when count()=0 & where I work with SQL Server, with a DB of a system ...
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1answer
23 views

Storage strategy for combination of all table items

Let's say I've one table with items with the following fields: id_item name description & one table with the user information, following like: id_user email name Now I'd like to let vote my ...
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33 views

narrow or wide - how to store huge amount of values

I think, I have two questions maybe in one (because they belong together and the first is priorized while the second is just an idea). The leading question is: Given billions of triplets (Timestamp, ...
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2answers
232 views

How do you determine the direction of a relationship?

Suppose I have the following relationship diagram: How would you describe the relationship determined by the red line? Reading from left to right you would say each order has only one customer so it ...
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1answer
41 views

Is it possible to enforce corresponding hierarchies?

I constructed the following toy example to illustrate my question. The left side of the schema describes a an abstract product consisting of its parts. The right side of the schema describes actual ...
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101 views

Can there exist Tables with no Non-Prime Attributes? If so, are they always normalised to 3NF?

Can there exist a table with no non-prime attributes? Here is an example of a table which I believe has no non-prime attributes: I came to this conclusion because the list of attributes in the ...
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1answer
40 views

Database design where % of rows have unused attributes/columns

Is there a theory/practice that promotes a design by analysing the usage of attributes of a table, such that given a threshold of unused columns over a percentage of rows would suggest that the ...
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74 views

Properties of grouping in relational algebra

The properties of relational algebra (commutativity, associativity, distribution) allow us to take a relational algebra expression and transform/rewrite it into another one which is logically ...
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79 views

Relational Algebra - Remove duplicate tuples without count

I was wondering if there is a way to remove the tuples from a table using relational algebra that have the same Value occurring in multiple columns or same column multiple times e.g. I want to ...
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Delete duplicate keys with relation algebra without count

Hi I am new to relational algebra. I was wondering if there is a way to remove the tuples from a table using relational algebra that have the same Value occurring in multiple columns or same column ...
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2answers
65 views

Delete duplicate keys with relation algebra

Hi I am new to databases and relational algebra. I was wondering if there is a way to remove the tuples from a table using relational algebra that have the same keys but different value. e.g. I want ...
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1answer
17 views

Creating a relationship between a document and a collection in MongoDB

I have no idea if this is even possible but I'd like to find out if there's some way to create a relationship between a document and a whole collection. I have a user schema that looks something like ...
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1answer
246 views

BCNF decomposition step without functional dependencies

I am performing the BCNF decomposition for the relation R = (A, B, C, D, E), that has the set of functional dependencies AB -> C, CD -> A, C -> E, and C -> B. After checking that the ...
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141 views

How functional dependency imply multi valued dependency?

I understand following informal definition of multi valued dependency: If multi valued dependency is α↠β, then whenever the tuples (a,b,c) and (a,d,e) exist in relation r, the tuples (a,b,e) and (...
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31 views

SARGABLE predicate must be in DNF. Why?

This famous paper about finding Access Path in Relational Database Management System by Selinger et al. says the following about criteria tree and predicates (at the end of 3rd section). A sargable ...
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1answer
35 views

Relational database field name design [closed]

Are there design patterns for naming table fields? For example, I've always used user (table)> email (field), but I see other well-used modes like user (table)> user_email (field). Are there any rules ...
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Using relational algebra to check a condition for all

I encountered this question while revising for my finals exam on database theory. The following database contains information about car repair workshops. The following tables are used: Given two ...
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4answers
360 views

Why is it considered that a set unnormalizes a database absolutely?

In order for a relationship to be on 1NF it needs to have all values as atomic, if there's a set it's not even in that first normal form: But intuitively I think that a table with that set would be ...
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1answer
83 views

See direct (instead of indirect) info about rented book in my “actors_directors” talbe dropdown list - Access365

I'm building a database of "actors_directors" who are renting "books" from my library - a kind of exercise to learn database basics :-) This is how my "actors_directors" table looks like. They are ...
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1answer
50 views

Entity Relationship Diagram - Entity depending on attribute

I have an ER-Diagram containing the entity employees. When employees have high skills they may supervise lower skilled employees. How can I model that in an ER-Diagram? My initial approach of was to ...
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1answer
111 views

Why is this relation not in Boyce-Codd Normal Form (BCNF)?

I have the relation (A,B,C,D,E). The relevant functional dependencies are: { E,B,C } → { D } { A,B } → { C } { E,B,D } → { C } { E,A,B } → { D } The candidate key is (E,A,B). I do not understand ...
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1answer
254 views

Finding all possible minimal covers

I have a relation schema R = {A, B, C} and the following functional dependencies: A → B A → C B → A B → C C → A C → B How many different minimal covers can I derive from this relation schema? I have ...
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1answer
131 views

What is a “part-key”?

I'm a beginner and student with regards to database design. The tutor recently discussed “part-keys”. I am struggling to find any information on what a part-key is, and I suspect it probably goes by ...
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1answer
38 views

How do I bring a relation/table to first normal form (1NF) when one of the attributes/columns can hold any number of values?

For example, MOVIES (movieid, movietitle) where movietitle can be any number of words, so I believe it's not atomic. How do you bring something like that to 1NF?
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1answer
352 views

Normalization with two potential primary keys

I have the following STUDENT relation in first normal form, where id has been identified as the primary key (email here is also unique for all tuples): STUDENT(id, email, first_name, last_name) I ...
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Design issue in transitive dependencies between tables

This is a made-up scenario, but I imagine that it can be of general interest, so I'll post it here. Imagine the following business rule: A course has a grade scale that determines what grades a ...
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2answers
202 views

Does a computed column break 3NF (third normal form)?

From my understanding of 3NF, there should not be any transitive dependency. However, this is what I found from the AdventureWorks2012 database Production.WorkOrder table. Since StockedQty is ...
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1answer
91 views

Show that functional dependency pseudotransivity holds using closure test

I'm currently studying a basic database course with the course literature Database Systems - The Complete Book1. In one of the book's exercises you are asked to Show that the following rules hold,...
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1answer
57 views

Determining minimal cover of a relation

I am having trouble understanding why a certain set of functional dependencies is not minimal. We have a relation R(A,B,C,D,E,F,G) with the following dependencies F: 1. A->CDE 2. B->FG 3. AB-&...
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1answer
2k views

Does using composite keys violate 2NF?

From all of the database related reference books that I have come across, there is a universal rule that state a database table should be at least 3NF. From what I understand, a table is considered ...
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1answer
153 views

Why should foreign keys be implemented in my relational database? [duplicate]

Earlier today, I sparked a rather unexpected debate with my professional peers on the topic of foreign keys in our company's databases. In a nutshell, a software engineer who is senior to me stated ...
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53 views

Relational Algebra: how to express the combination of aggregation and projection?

Here I have a table, I would like to query the average salary of departments, the SQL statement should be select dept_name, avg(salary) from employee I try to express this SQL in relational algebra....
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1answer
61 views

Can and should relational database contain tables that don't have primary keys? [duplicate]

So this site says: "All tables in a relational database should have a primary key". Here it also says that every table should have a primary key. But if I look here and here they say that tables ...

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