This style of notation can also be used with polyheirarchies (items w/ more than one parent), if you leave a couple of reserved characters for the notation. Here's an example that my teacher gave us for a class on thesaurus design: 
Facet A : Subject
  A1 : Science
  A1.1 : Physics
  A1.1.1 : Optics
Facet B : Grade Level
  B1 : High School
  B1.1 : 10th Grade

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So, '10th grade optics' would be 'A1.1.1B1.1'. We use a delimitor between items, so we aren't restricted to subsets that each have a unique character in them, allowing for wider branching. (depth is still a problem when dealing with fixed line lengths)

The problem is, it's fine for humans to understand, but a little messy to sort on in SQL. You can get around this by adding another delimiter between facets, and ending every notation w/ a null heirarchy: ';A1.1.1.;B1.1.'. We can then get all items relating to 10th grade physics w/ LIKE '%;A1.1.%;B1.1.%', without fear of getting 'A1.10AB1.1', and still getting items that may have even more facets, eg a movie for 10th grade physics ';A1.1;B1.1;C1'

The only requirement is that you keep each of the facets in order. (what order doesn't matter, so long as you're consistent)

If you'd like a more complex example:

If we were to try to categorize cooking implements:

Facet A : Use
  A1 : Utensil
  A1.1 : Spoon
  A1.2 : Whisk
  A2 : Pan
  A2.1 : Roasting Pan
  A2.1.1 : Covered Roasting Pan
  A2.1.2 : Uncovered Roasting Pan
Facet B : Material
  B1 : Metal
  B1.1 : Stainless Steel
  B1.2 : Aluminum
  B1.3 : Cast Iron
  B1.3.1 : Enameled Cast Iron
  B2 : Ceramic
  B2.1 : Glass
  B2.1.1 : Pyrex
  B2.2 : Stone
Facet C : Size
  C1 : 1 qt
  C2 : 2 qt
  C2.1 : 2 qt, shallow

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Therefore, a standard 9x13 pyrex dish would be ';A2.1.2.;B2.1.1.;C2.1.', and we could find all 2qt glass pans with '%;A2.%B2.1.%;C2.%'

In reply to Re^2: Trees in SQL, adjacency lists and sorting. by jhourcle
in thread Trees in SQL, adjacency lists and sorting. by BUU

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