The relational-theory tag has no wiki summary.
33
votes
5answers
2k 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 ...
12
votes
3answers
775 views
Why does ANSI SQL define SUM(no rows) as NULL?
The ANSI SQL standard defines (chapter 6.5, set function specification) the following behaviour for aggregate functions on empty result sets:
COUNT(...) = 0
AVG(...) = NULL
MIN(...) = NULL
MAX(...) = ...
3
votes
1answer
93 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 ...
2
votes
2answers
150 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 ...
2
votes
1answer
518 views
Check if relation is in BCNF
I have the following relation and function dependencies.
R = ABCDE
F: C->AB, D->A, BE->CE, E->B
I have to check whether this relation is in BCNF.
Wikipedia states:
...if and only ...
2
votes
0answers
117 views
Transform XPath map into XML document using relational data
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 ...
1
vote
1answer
81 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,
...
1
vote
1answer
85 views
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 ...
1
vote
0answers
18 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 ...
1
vote
1answer
68 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 ...
1
vote
1answer
68 views
Relation candidate keys
I have the following relation:
R = ABCDE
F: AB->DE, AC->B, DE->C, AE->C, C->AB
I need to determine this relations candidate keys.
Now, if I understood the defintion correctly:
...
0
votes
1answer
27 views
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 ...
0
votes
3answers
53 views
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 ...
0
votes
1answer
389 views
Is this partial dependency?
I have this relation:
R = ABCDE
F: AB->DE, AC->B, DE->C, AE->C, C->AB.
Is AC->B a partial dependency in F?
I'm guessing it is, because
C->AB can be decomposed to
C->A
C->B
...
-1
votes
1answer
77 views
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 ...
