| Re: Traversing subtrees [message #3647 is a reply to message #3645] |
Sat, 06 April 2013 06:32   |
AJM
Messages: 2368
Registered: April 2006
Location: Surrey, UK
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Senior Member |
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Those tables *ARE* normalised. If you think they are not then please identify which Normal Form they are violating. Missing a "well-defined interpretation" is not in any definition of any normal form.
It is quite clear to me that you haven't understood the types of hierarchy that my tree structure is supposed to represent. My article clearly shows that it deals with an organisational hierarchy with different levels such as COMPANY, DEPARTMENT and SECTION where there can be a number of entities at each of those levels. The rule is that any entity below the top level can have only one parent, and that parent must belong to next highest level. I designed this structure decades ago for a payroll system where people at a certain level in the hierarchy could only "see" the records for those people who were their subordinates in that hierarchy. This structure made it easy to add levels, to maintain the list of nodes that existed at each level, and to link (including unlink+relink) a person with any node in that structure.
If you try to use this structure to represent anything more complex, such as your itinerary example, then you are wasting your time. You will have to do that using separate tables with specialised relationships.
Tony Marston
http://www.tonymarston.net
http://www.radicore.org
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