Tagged Questions

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 ...

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How to use a relational database to store all users who belong to a group, and all groups which a user belongs to?

This is a beginner question: A user can belong to many groups, and a group can contain many users. Let's say the two tables look something like this: user_id user_name user_email group_id ...
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Person-Address = 1-1 or N-1 (In this context)?

I have 4 tables: Province (province_id, name) City (city_id, name, province_fk) Another Smaller subset under city say... City_sub (sub_id, name, city_fk) Address table (address_1, address_2, sub_fk, ...
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Suitable schema to record email sent and response received and invoicing in DB

When a form submission is made on a website: An email with form's details is sent to a set of recipients email addresses stored in a DB. The email contains a link for the recipient to show interest. ...
277 views

Convert query in words to relational algebra

I have a follow-up question to a question I have previously asked on SO. Instead of the query from my initial question, I want to convert the following into relational algebra: List the names and ...
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Two ways to define may-have-a (1..0-1) relationship

Today I have discovered for myself that may-have-a (1-0..1) relation can be implemented in two different ways. Let A be the "main" table (left side of ..) and B be the "linked" table (right side of .....
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How to model that several variants of a merchandise item may affect its price?

I am an absolute beginner to relational data modelling. I hope my question will be acceptable here nevertheless. Let’s say there’s a merchandise. An item of merchandise may have variants. (While it ...
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How does one map nested multi valued attributes?

Let's say one has an Entity E. E has a multivalue Attribute M1 and a simple attribute A1. However M1 consists again of a multivalue Attribute M2 and a simple Attribute A2. How would one map this ...
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Hierarchial, tree-like data model, which DB type? [closed]

I have the need for a database, I know what to store, however the question is what database to use and why. I have used relation SQL databases a lot in the past (up to a few years ago) and I know I ...
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A tuple relational calculus equivalent for the given SQL query

well here's a SQL query select Employee.Name,Department.Name from Employee,Department where Employee.Dept_no=Department.No I have tried a tuple relational calculus on it as {t|Ǝ s ∈ Employee (s[...
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Difference between an Entity and a Relation

I am currently reading about entities and relations and I can't understand the difference between them? According to Wikipedia we have an example of a relation like in the pic: This looks exactly ...
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How can I represent a list of words and related words in a relational structure?

I have a list of header words and words as below and I am trying to understand how best to represent these in a relational table: Header Other Words Words abandon abandoned, ...
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Estimate result size of Natural Join

I have the following tables R1(A, B, C) with 1.000 rows R2(C, D, E) with 1.500 rows R3(E, F) with 750 rows where a bold letter denotes a primary key. I need to estimate the number of rows of the ...
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Data inconsistency prohibition if a table refers to another via two many-to-many relationships

I have following database design (by => foreign key constraints are depicted): Company[id] CompanyRealm[id, company_id=>Company.id]many2many Project[id, company_id=>Company.id] ProjectRealm[id, ...
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Best data modelling approach to handle redundant foreign keys in relational model

I am looking for advice on the best relational modelling approach to store surveys, questions, and responses. I am looking for which of the two approaches below looks best, or an alternative approach ...
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What's the use of assigning foreign keys on MySQL [duplicate]

Aside from indexing the value of the foreign key, what is/are the advantage of assigning foreign keys to databases? I've seen a lot of databases that does not assign foreign keys, but is relational by ...
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How to express functional dependency in relational calculus and SQL?

I want to express in relational calculus and SQL the following functional dependency associated with Vote. This is the first time I try to implement it in actual SQL and relational calculus. Vote: #...
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What a closure `F` of a family of functional dependencies is?

I have some issues to determine what a closure F of a family of functional dependencies is. I know its definition is F⁺= {X → Y\F⊨X → Y} One example Let be the following relation R(Student,...
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Dependency preserving decomposition

Let be the following relational scheme: F={AB → C; AC → B; AD → E; B → D; BC → A;E → G} Is the following decomposition without loss of dependency? R¹=(A,B),R²=(B,C),R³=(A,B,D,E),R⁴...
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What normal forms are these two relations in?

I have a set of relations {A, X, Y, Z}: Given that the functional dependencies: AX→Y, AX→Z, Y→A, Z→X, I would think that this relation would be in the normal form 1NF, because there is a partial ...
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Creating relationships between tables that where not specified in the given UML diagram

Good evening! I'm working on designing my very first actual database from the following UML class diagram (in French unfortunately): I'm at the point of creating the relational sketch of it which ...
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What are the conditions for a functional dependency to exist?

Let be a relation R of scheme: R(A,B,C,D,E) and r an instance of the relation. What functional dependencies verify r? Observing the functional dependencies that r verify are doesn't show that they ...
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Decomposition In Boyce-Codd Normal Form may lost relations?

Let be the following Realtion R={UtilisateurID, Nom, Prenom, AdresseEmail, Login, Passwd, ServeurMail} with the functional dependencies: F = { UtilisateurID → Nom, Prenom UtilisateurID, ...
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Why does 3NF Normal form allow transitive dependency of prime attributes?

I'm just a beginner in Relational Design, When i came across the definitions of 3NF and BCNF. According to 3NF, Any non-trivial functional dependency X-> A shouldnt exist unless X is a super key or A ...
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Is this table in third normal form?

I have a table L(A, B, C). Functional dependencies are: A->B, A->C, B->A, B->C. Is this table in 3nf? My thoughts: A and B are keys of this table. Table is in 3nf if there are no transitive ...
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Modeling a database with UML scheme and every type of relations

Good evening! This is my second database designing, the first one can be found here. I want to find the relational scheme of bridal registry database given in the following French UML graph: I ...
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Algorithm of decomposition in Boyce-Codd Normal Form

I have to give the keys of the following relation, say if they are 1,2 or 3NF and give a BCNF decomposition. Yet I think I misunderstand the difference between 2NF and 3NF: R(A,B,C,D,E) with F¹={A→B;...
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Grouping operator in DBMS (γ) is duplicate-impervious. Why?

I'm reading Database Systems by Ullman where its mentioned that γ is duplicate-impervious. I don't understand why a simple group operation should cause elimination of duplicates. Aliter, Duplicate-...
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How to decompose this relation into 3NF relations?

Today I read about the 3NF decomposition algorithm. It said: Find a minimal basis of F, say G For each FD X → A in G, use {X, A} as the schema of one of the relations in the decomposition ...
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Decompostion with loss of information shown by junction

F={A → B; B → C; C → D} R is decomposed in R¹(A,B) & R²(A,D) & R³(C, D) Is it without loss of information? I would have said yes as far as with the following array A → B x²²= b B → C x¹...
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Decomposition and loss of dependencies

I'm lost in the sea with the following example of decomposition without loss of dependencies... Let be the dependencies F= {A → BC, C → AD} R is decomposed in R¹(A,B,C) & R²(A,D) I want to know ...
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Multiple columns from one table rely on one single table

I'm working with PostgreSQL 9.2 and have the following relation between 2 tables: The table town stores a list of town codes. The table perimeter stores perimeters denomination. Towns are almost ...
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How can I represent discrete ages in a relational database?

I am using PostgreSQL to store data on a Person. I need to store whether each Person is capable of teaching a Child of each age year, from 0-17. The ages are discrete values, and a Person can have any ...
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Many-to-many tables tracking participation at multiple events

I am working on a side project to track observations of different birds, and the time spent during each observation. Right now I have a table called Observations: obs_id | start_time | end_time ...
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Grouping in relational algebra with more than one grouping attribute

So I am having a bit of trouble understanding the Grouping operator, Ɣ. If had a the relation Sales(employeeId, date, salePrice) and I wanted to list the largest salePrice for each employee per day, ...
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Explaining how to design a database to model the stocks of a firm to my Grand'ma

My teacher always asked us to create simple but precise databases. Being able to explain it simply but not without leaving the theory "to your wise but aged loved Grand'ma"... I have to create a ...
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Find missing associations in many to many relationship [duplicate]

I have client and category tables with a many to many relationship via client_category_association. table: client id name 1 ek 2 do table: category id name 1 un 2 dos 3 tres table: ...
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Lost (and found?) on 4NF

I'm so extremely lost on 4NF it's not even funny. I've been reading on it for literally days and, except for about 15 minutes ago, I wasn't sure that I had even broken the surface. I think I might ...
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What is the difference between a field and a column? [duplicate]

Although I have always regarded the terms as interchangeable, I have read somewhere that they’re not the same. I suppose this means that a record is not exactly the same as a row either. I teach that ...
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First Normal Form, why is it good and how does it reduce redundancy

I asked a similar question on SO and was advised to ask the type of question here. This is for a course on relational databases. A sample problem asks "how come every table in a relational database ...
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Registration Schema Implementation

I'm building an online app that is going to handle reservations. Individuals making reservations can make one for a single day or multiple days. I have come up with two schema and cannot see much of a ...
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Why are multivalued dependencies named that way?

I'm doing a relational design course on Stanford open courseware. I understand why functional dependencies are named that way - A->B means B is specified when A is specified, or in other words, the ...
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Multiple relationships between the same entities

Is there a name for when 2 entities can have multiple relationships between them?
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Counterexample to demonstrate there are no functional dependencies

For instance in the following example Let be F={AB→C, B→D, CD→E, CE→GH, G→A} Do we have AB→G? We don't have any functional dependencies. I am able to show a functional dependency when it ...
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Are three equalities on a join operation legal in relational algebra?

Let be the following database: Film(Title,Director,Actor) Produce(Producer,Title) Express in relational algebra the following questions: Which actors produce at least a Film they directed? ...
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How to normalised a relation using fds

G = {{A,B}→{C},{B,D}→{E,F},{A,D}→{G,H},{A}→{I},{H}→{J} }. Which is a key attribute? {A,B} or {A,B,D}?
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how to represent what functional dependencies are?

I'm trying to understand how to represent what functional dependencies are: A1, A2,..., An → B If two tuples of R have the same attributes for A1, A2,..., An then they have the same value for ...
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If my understanding of BCNF is correct, how did we know that R1 was not in BCNF?

Given that R(ABCDE) AB->C BC->D CD-> E DE->A Since, B is the only attribute absent on the right-hand side, we would start with B. {B}+ = B . Not a CK. Next, we ...
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Cartesian product and Division in Relational Algebra

Let be the following database sketch: Cineroom(Name, Hour, Name) Film(Name, Director, Actor) Saw(Spectator, Name) Produce(Producer, Name) Likes(Amateur, Name) How to express, ...