1
vote
1answer
108 views

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 ...
-1
votes
1answer
38 views

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 ...
2
votes
1answer
90 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. ...
1
vote
1answer
166 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, ...
2
votes
0answers
307 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 ...
2
votes
2answers
1k 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 ...
16
votes
3answers
2k 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(...) = ...