# Tagged Questions

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18 views

### Explain Database Normalization Forms with examples? [on hold]

I am a newbie in databases. I am learning database theory using SQL Server . I have read lots of material on normalization/de-normalization but never understand them clearly. Can you please explain ...
83 views

### Best data modeling approch to handle redundant foreign keys in relational model

I am looking for advice on the best relational modeling approach to store surveys, questions, and responses. I am looking for which of the two approaches below looks best, or an alternative approach ...
28 views

### Storing shipping details in orders table

Have orders table and among columns the following: shipping type, shipping service provider, delivery type and shipping address. Let me explain what these are. User can choose between two shipping ...
17 views

### Calendar mult-data mult-request relations model

I have a table "USERS" I have a table "HOUSES" I have a table "CALENDAR" I have a table "TRANSACTIONS" I need build a schedule(CALENDAR) that relates to every home, every home has days available for ...
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### Tuple calculus: unsure if my reasoning is correct

I'm currently working through a tuple calculus question and I ran into a situation where I have two answers that seem correct to me and I am unsure if my logic is wrong or if they are both correct ...
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### If a table with a surrogate key has a column known to have unique non null values (eg SSN), does it violate 3NF?

As I understand it, third normal form (3NF) basically means there should be exactly one key. If a table with say an auto-increment id column also has a column known to be unique and not null, eg ...
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### Select all from A where attribute exists in B [closed]

I have a question involving relational algebra. Suppose I have two relations A and B, where B has a foreign key that references A. Using relational algebra, how do I select all tuples from A where ...
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### What are the practical uses for relational calculus?

In my Database Design course we are learning both Relational Algebra and Relational Calculus. I can see where Relational Algebra could be useful since it is closely tied to SQL. Our professor said ...
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### Explaining 2NF vs 3NF with an example

I have a problem with second normal form (2NF) and I have been unable to solve it by using Google. It is making me crazy because I am a teacher and I don't want to teach wrong stuff to my students. ...
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### Method to economically insert Bill of Materials BOM items into a Stock Table?

I’m new to the wonderful world of database design and I’m currently experimenting with an implementation for my little brother relating to Model Train Sets. [I will be using SQL Server 2012 / Asp.Net ...
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### What are the practical reasons behind learning relational algebra?

Learning how to formulate query-like expressions in relational algebra is a traditional part of many, perhaps most, "Introduction to Databases" courses. This is usually justified by the assertion ...
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### Table with relationship to multiple tables

I want to persist contact details for persons. This contact details can belong to a client, or a next of kin member, or whatever other entity. I'll be using Doctrine2 in a Symfony project, that will ...
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I am not sure whether this is offtopic. However, please read on. A person whom I know has encountered a problem in the answer key published by CBSE, India. As their key challenging form contains a ...
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### The key of the relation resulting from a full outer join

What is the key of the relation resulting from a full outer join? If there is no key, how is it said that the relational algebra is closed under its operators and the result of each operator is also a ...
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### Using logic or in relational algebra

In relational algebra I can obtain the courses taught in Fall 2009 or Spring 2010 with Π_{course_id} (σ_{semester=“Fall” Λ year=2009} (section)) ∪ Π_{course_id} (σ_{semester=“Spring” Λ year=2010} ...
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### How to enforce that one row in a parent table MUST be associated with at least a row in a child table [duplicate]

I have a one to many relationship enforced via FK, classic example order and order_item, whereby a fk on the order_item to the order table enforces that one item belongs to one order, and one order ...
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### Translating duplicated data entries into Domain Relational Calculus

Given a following table named BOOKING, which contains three columns, guest_name, room_number and booking_date, how do I find the all the guest_name that have the same room_number and booking_date? I ...
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### Tuple Relational Calculus help? [closed]

I have the following three questions in relation to the following diagram: 1) How many copies of the book titled “The Lost Tribe” are owned by the library branch whose name is "Sharpstown"? ...
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### Is there a standard direction to read cardinality relationships in?

I'm reading some documentation, and before getting egg on my face for questioning it, I'd like to check the standard way of reading UML cardinality relationships. The docs I have look like the image ...
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### expressing constraints in relational algebra

I need to express this constrain in relational algebra: I have some table with one column with all possible values: ALL_VAL and table with some values from ALL_VAL that not mach some rule: ...
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### In terms of relational databases is a Relvar the equivalent to a Schema?

I've been reading up on relational databases and the term Relvar and schema keeps coming up. From my understanding: A relvar is a variable holding a relation. A schema is a logical definition of a ...
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### Relational Algebra Queries - How to Approach Problems?

I have the following schema: employee(sin, emp_name, emp_address, salary, supervisor_sin, dept#) works_on(emp_sin, project_number, hours) The question is: Assume that no employee works on more ...
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I am trying to make sense of an example of 3NF decomposition using the 4-step algorithm mentioned by Ullman here, but I'm not understanding what my lecturer is doing with the last step (or, worse, I'm ...
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### Database relationship refresher

All, After several years of programming applications, I'm looking to refresh my understanding of databases. What I knew to begin with was fairly basic, and honestly, it's disappeared over time. I ...
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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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### Algebra of joins, is there ever a need to repeat a table in a series of joins?

First, I'm just asking about inner joins. And, I'm thinking self joins or reflexive joins through a join table are special cases which depend on how the data is structured (e.g. city - state - ...
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### Logical optimization of joins and selections

I am studying query optimization. I have read that it might be preferred to apply relational selections before a logical join operation. Is there a scenario where joining first and then applying ...
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### autonumber and one-to-many relationship

I have a main table with an autonumber key. I need to introduce a new table, related to the main table in a one-to-many relationship (i.e. a record in the main table could conceivably link to several ...
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### What is the difference between a union and an “isa” disjoint relationship?

To me, it seems that a union and an "isa" disjoint relationship are the same. What is the difference between the two?
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### Keys of a relation

I got relation: R = { (ABCDEF) , (A B F -> C, B C -> F, F -> A, F -> B, D E -> E, E -> D)} keys of this relation are: {BCE}, {EF} But i dont get it... why? Why not only {EF}.. it got less ...
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### Find the highest graded student, using Tuple Relational Calculus

Suppose I have a table of students, containing their ID and their grade: ----------------- | id | grade | ----------------- | 1 | 83 | | 2 | 94 | | 3 | 92 | | 4 | 78 | ...
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### An example of a relation without any functional dependency?

I have to find a really bad example where you cannot apply Heaths Theorem because of a missing functional dependency. But am I right, that every relation has at least one functional dependency? Or ...
128 views

### Estimating a Size of Joining a Relation with itself

I'm studying size estimation of logical query plans in order to select a physical query plan. I was wondering what is the size of joining (natural join) a relation to itself? e.g R(a,b) JOIN R(a,b)
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### Relation that combines entries with their subsequent entries, in order of date

I need to come up with a query that can do the following: I have a table with a bunch of transaction records, whose attributes are not important, other than the attribute Date. I need to come up ...
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### What do you call a relationship between two entities without a foreign key?

Let's say I have two tables that are related in that one table contains a field that is a key to the other table. In other words, it would be a 1:1 or 1:* depending on constraints. Let's call it an ...
889 views

### Event Management System

I'm working on a "Event Management System" Online Software Project where the system requires the following: Viewers can sign up for their user account Events can be created by the admin from the ...
116 views

### How to select nodes where all children is satisfied?

I have a tree structure of light bulbs. I want to turn on all the light bulbs starting from the leafs of the tree. A light bulb cannot be turned on unless all its immediate children are turned on. ...
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### Referencing a Join table

I have a relationship that I am trying to represent in a database. Imagine I have series of many to many relationships... Say student to proffesor, and professor to classes... A student can many ...
240 views

### Separating tables vs having one table

At the moment I have a table setup that looks somewhat like this: create table tbl_locationcollections ( id int(11) PRIMARY KEY IDENTITY, --(Primary Key), name varchar(100) not null, ...
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### Column partially determining accepted values of another column, should be somehow normalized?

I have a table that describes a set of properties (yes it's metadata, in this case there's a valid reason to have it in the database); among other things I tell the type of the data that can be ...
428 views

### Transform XPath map into XML document using relational data [closed]

Background Most modern databases have XML functions that can be used to extract data in an XML format. I want to avoid the task of manually calling XML functions to extract the data. This problem ...
2k views

### How does ORDER BY really work? [duplicate]

I have one question that confusing me. By logical query processing, the SELECT clause comes before the ORDER BY clause, and in theory, every step in a query creates a virtual table. So, how is it ...
550 views

### Closure of attributes definition clarification

A databases book I am using states [1]: The closure of {A1, A2, ..., An} under the FD's in S is the set of attributes B such that every relation that satisfies all the FD's in set S also satisfies ...
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### What is the real-world purpose of an optional 1:1 relationship?

I know that one-to-one relationships can be used to split data into multiple tables for performance or security and that it is used to create a is-a-relationsship. But aside from that things, what is ...
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### Approach to Analyzing Database Requirements

When presented with requirements for a database driven system, I always find it useful to imagine a world with no computers, where data records were kept in filing cabinets and data was collected by ...
10k views

### Why can't RDBM's cluster the way NoSQL does?

One of the big plusses for nosql DBMS is that they can cluster more easily. Supposedly with NoSQL you can create hundreds of cheap machines that store different pieces of data and query it all at ...
249 views

### Symmetric self-referential many-to-many

I'm trying to model a hierarchy of categories where a category can have multiple parents ( an overlapping tree model as described in this book ) I have the following tables video_categories int ...
396 views

### Configuration options for relational algebra evaluator “ra.jar”

I am participating in the Stanford Database Class, and there are some problem sets to be solved in "mathy" relational algebra syntax. The course instructors demand from students to use this ...