Questions tagged [relations]

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Proof Full Outer Join associative and Commutative

I'm trying to prove to myself that the order of full outer joins doesn't matter, but, in an abstract sense, I'm coming up with nothing. How would one go about proving that the order of a set of FULL ...
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1answer
18 views

Structure to represent matches between teams or individuals

Say there's a sport that can either be contested between two individuals, or two teams comprised of several individuals. So obviously we need the standard players, teams and teams_players tables. But ...
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1answer
38 views

Database model / design best practice

I'm looking for a some guidance/advice on how to best model my data. Some background: I have location information that I'm trying to model. There are two categories of locations (company and non-...
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1answer
64 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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74 views

Are these tables in 3NF? [closed]

Update: I updated the question to be more specific and also updated the diagram. I am currently in my last semester, I did use SQL before then I left for MongoDB but the practice project requires to ...
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1answer
35 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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35 views

MONGODB -> Whats the best DB model for friendship user Relationship

Good morning: I want to implement friendship between two users, that friendship is defined by a status (accepted, pending, blocked). It only occurs to me to use a collection as follows: USER1 - USER2 ...
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3answers
54 views

Given a table containing class and students, need to find the diff in students

I have class_students table where given 2 class ids I need to find the diff in students. This is sql fiddle that I used, http://sqlfiddle.com/#!9/dba48e/11 Any help here.
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1answer
18 views

three tables relation order is correct but values are false

i have those tables table1 | id | name | | 1 | axe | | 2 | bow | | 3 | car | | 4 | dart | table2 table3 | t1_id | ...
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1answer
49 views

Database design for Scavenger Hunt Game

I'm designing a section of my database which links User, Team and Hunt. I would like users to be members of a team but the team can play multiple hunts (so can be used multiple times). The first idea ...
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18 views

Database design for arrays - utilize relational database

I am trying to build an application, where users can create unlimited "parsing fields", that will contain either a: String Array id | name | type ------------------- 1 | name | string 2 | table |...
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141 views

Is a relation with two attributes always in Boyce-Codd Normal Form (BCNF)?

I went through some tutorials on BCNF and got that a relation with two attributes is always in BCNF. My question What if a relation with two attributes has no prime or non-prime attributes? That is,...
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A multiple choice question, with multiple or only one correct answer, and sometimes no correct answer

It's a question & answer based model, and here is the scenario: A question with multiple choices given, with 3 to 7 correct answers among those choices. A question with multiple choices given, ...
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2answers
137 views

Are Cataloge and relation cache in PostgreSQL per connection or global to the server?

I am trying to figure out if Postgres' relation and catalog cache are per connection or per server? I did find that plan caches are per connection. It would make sense for the cache to exist for as ...
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31 views

Database pattern with two types of owners for an object. Like how users and groups can make posts in FB [duplicate]

Please help. There is a problem I have tried to resolve for some time now. I want to come up with a pattern where i have a table with advertisements on a web platform. These advertisements shall be ...
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1answer
69 views

Table Relationships for Property Bags Configuration

I am creating a database table setup for user preferences in my application based partially on this design and while I created the tables and foreign keys, I can't seem to figure out the relationships ...
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1answer
296 views

Is it good practice to create a separate table for views count?

I have created a blog table where I'm having a field called views_count but I have heard that updating the views_count field on each page view is strain. So I have created now a seperate table for ...
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1answer
25 views

Creating blog fresh tables from old tables [closed]

I am re-creating a database structure from old tables. New tables should be optimized and easy to understand. In old database the developer used video table name for storing video blogs. However I ...
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1answer
198 views

BCNF decomposition does not prevent insert anomaly?

I am in the process of designing a schema for a simple polling app and have tried to normalize it with the decomposition algorithm from this lecture. Requirements A poll has a unique id (poll_id), a ...
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1answer
44 views

Recursive Relationship [closed]

I have an application, which contains two Models Question and Option Each Question has many Options Each Option may have many Questions (and this Question may have many sub Options) so on and so ...
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1answer
44 views

mysql join many to one table and display each 'many' record as an additional column

I have a many to one table. For example: user_id, name 1, chris 2, john and user_id, item 1, apple, 1, banana, 2, orange How could I run a query that returns something like this: user_id, ...
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61 views

Normalizing a relation schema with 'abstract' attribute names

I'm a student currently attending a database design course and I'm having some issues normalizing 'abstract' attribute relation schemas from 1NF all the way to 3NF. When we used examples with '...
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1answer
80 views

LOB in frequently queried table or separate table

I have a table named FinTrans in an Microsoft SQL Server 2012 server with approx 10K-30K records at any given time and about 2K records are inserted and deleted each day. The table has 3 nvarchar(...
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2answers
242 views

Is the inverse of Armstrong's Axioms valid as well?

I have been looking into Armstrong's axioms a little bit. In a homework[1] exercise I was asked to prove A→G is in F-closure. I managed to get it at this point: AB → GB Being at this point, can I ...
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1answer
37 views

Show multiple joined rows

I have a simple relational MySQL database with users, cars an a log table. Everything works fine. In the log table I want to 'register' the users of a car. But a car can have two users at the same ...
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3answers
75 views

Constraining data based on related data in a different table. Should this be done on the database level or the application level?

I am working on a database that contains our customer information as well as information about our vendors. Part of that includes matching our customers with our vendors, the account numbers those ...
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1answer
596 views

Circular dependency between 2 table with one to one and one to many relationship

I have 2 tables in SQL database, User and Company. User belongs to one company. So user table has UserID & CompanyID (foreign key) Company can have multiple users. One of them will be the ...
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1answer
108 views

Table with relation to two others (three-way relation?)

I stumbled across a weird situation that one table has relation to two others but record in it can only have a relation to one of the tables. To clarify things, I have tables: attribute, visit, ...
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1answer
94 views

Difference of sets with different amount of columns?

Problem When there exists two sets, R and S, and we take the difference R - S, we remove the elements from R that exist in S. For cases where the number of columns are the same on both sides, such ...
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2answers
9k views

Is it a bad practice to have several mutually exclusive one-to-one relationships?

Say, a table car has one-to-one relationship to tables electric_car, gas_car, and hybrid_car. If a car is electric_car, it can no longer appear in gas_car or a hybrid_car, etc. Is there anything ...
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1answer
822 views

What is the definition of bad circular dependency?

I have been searching for while for a good resource, where the circular dependency is well explained, unfortunately nothing good found. Therefore I tried to understand exactly which kind of circular ...
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1answer
56 views

One to Many Relationship: Describing the Tables

If we have two tables in a One-to-Many relation ship, is there a standard name to properly refer to the tables? For example if I have a table of books, and a table of authors (ignoring for now the ...
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145 views

DBMS Query Preprocessor Checks Semantics or Syntax?

I encountered the following statement when reading about the constituents of a query processor in a DBMS: ... (b) A query preprocessor, which performs semantic checks on the query (e.g., making ...
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3answers
2k views

Return “parent” rows where only one “child” record exists in one-to-many relationship

Having a one-to-many relation, I am trying to get records in the accounts table where only one record exists in the accounts_prop table. I have the following query, but it does not do the desired. ...
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1answer
457 views

Database relationship inheritance [closed]

I have a vehicle sales store. In my store I sale many kind of vehicles for example cars and boats. And I would like to keep track of the sales. My vehicles has some attributes as: Arrive on store ...
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1answer
455 views

Mysqldump subset of database with relational data using a master table

I have found a lot of articles describing the concept of exporting and importing relational data, but I have not been able to find any article describing this more generally for a MySQL database, ...
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2answers
58 views

3 different queries from 2 tables

I have three tables (palettes, boxes, layout) with sample values: Palettes |ID_P|sign| | 1| P1| | 2| P2| | 3| P3| | 4| P4| Boxes |ID_B|sign| | 1| B1| | 2| B2| | 3| B3| | 4| ...
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0answers
2k views

How do you perform the chase test?

Trying to follow the chase algorithm as I understand it, If two rows agree in the left side of a functional dependency (FD for brevity), make their right sides agree too. Always replace a ...
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1answer
69 views

Equivalent functional dependencies

I cannot understand why F1 is not equivalent to the other sets of dependencies below given the relation R(A,B,C): F1 = {A → B, B → C} F2 = {A → B, A → C} F3 = {A → B, AB → C} This is question no. 6 ...
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3answers
784 views

Contacts table with multiple entries?

Here is a part of our Database: Table: Users ID INT First Name VARCHAR Last Name VARCHAR Email Address VARCHAR ... Table: Contacts ID INT User_id INT (Foreign Key)...
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2answers
3k views

Counting distinct entries in a column using relational algebra

So I have a table similar to this one. Each user has posted a review about one or more hotels(A,B,C,D) but on different dates so there are no duplicate tuples even though a person might have reviewed ...
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1answer
70 views

MS SQL DB design help

I'm stuck with table design and query FOR XML Sync engine for Ads requires XML schema in this format <attribute1>value</attribute1> <attribute2>value</attribute2>… <...
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5answers
451 views

What is the best practice for selecting from two tables with multiple relations?

I have two tables with a structure like below: and am selecting the data with below query: SELECT (SELECT Name FROM Management_EnumerableItem ME WHERE ME.ID = DN) DN, (SELECT Name FROM ...
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1answer
1k views

Proof that Inner Join is Commutative

I'm trying to prove to myself that the order of inner joins doesn't matter, but, in an abstract sense, I'm coming up with nothing. How would one go about proving that the order of a set of INNER ...
0
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1answer
78 views

In a school database, do I need to have repeating data when a student can also be a teacher?

I've seen lots of ERDs of schools, teachers, and all the junction/associative tables. They always have three tables: Teachers Students Associative table betweeen them, maybe subjects of something ...
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1answer
666 views

Functional dependencies proving method

I was having some difficulties when trying to use Armstrong axiom to solve functional dependencies. R (A,B,C,D,E,F) Given FD = { A->CD , C->BE, B->F, F->A } Prove A-> BF By using ...
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1answer
41 views

DB architecture: What if a table is part of another table

My SQL Server database has the initial table named Lead. This table has all kinds of info about a lead person. It has 3 foreign key columns: IndustryId, BusinessTypeId and ReferenceId. I have a ...
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80 views

Table Relationships: Table 1 (Field 7, Field 8, Field 9) to Table 2 (Field 1)

I'm sure that this is probably 'Databases 101', but I can't seem to figure it out. So, I'm setting up a database in Access - that I will want to move to MySQL, and I currently have 4 tables, set up ...
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3answers
73 views

Meta-relationships (Relationships between relationships) versus tuples

I am designing a database which needs to store some fairly complex relationships among many different types of entities. In the simplest example, let us say I have three types of entities - "Student",...
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2answers
77 views

Normalization of a table

I am trying to normalize the following table but having some difficulties Mem_id mem_name phone email Book_num Book_titl Author_nam publication edition 100 smith 12443 eml1 200 physics ...