Jeff Moden (3/10/2011)
Wesley Brown (3/10/2011)
I prefer to use a two column approach to build a tree instead of the single column style that you have to split, it doesn't scale well. For smaller stuff like this HR representation it should be OK. Trees and Hierarchies in SQL from Joe Celko is one of my all time favorite books! Good article!your brother,
Phil McCrevice
If, by "two column approach", you mean "Nested Sets", I absolutely agree. In the larger article I'm writing on hierarchies, I'll actually demonstrate a new method for converting an "Adjaceny List" to a "Nested Set" that I think you'll like especially since it gets away from the RBAR of a "push stack" to build the "Nested Set". Ben-Gan also has an alternate method for doing the same thing.
You'll also like the "warehouse" table that I'll build in the coming article which you might prefer to a "Nested Set" because the "warehouse" table contains preaggregated answers for the 4 of the more common hierarchical lookups that people seem to do on a regular basis.
I've got a two-step script that can generate a nested sets "path" from an adjacency hierarchy, turning it into a nested sets hierarchy, using Cross Apply and a ranking function. Single query, and much more efficient than the method Joe originally proposed (but also uses features he didn't have when he originally wrote the article).
I've found you can also short-circuit the conversion and get really good performance using the ASCII table and a binary colation for your set headers and ranges instead of numeric values. You don't need the binary colation for smaller hierarchies, but it comes in handy for bigger/complex ones with lots of top levels. (This method takes advantage of the fact that you can, in the recursive part of the CTE, concat a string together based on level and row_number within the level, and that "AAAB" is between "AAA" and "AAB" in a string sort. Much simpler than using integers for the ranges.)
The string-range version is a bit of a pain to set up in the first place (making sure all top level and sub-level strings are guaranteed unique out of a limited dataset of ASCII characters that can be sorted on), but works nicely once it's in place.
I'm interested in seeing how your article solves it.
- Gus "GSquared", RSVP, OODA, MAP, NMVP, FAQ, SAT, SQL, DNA, RNA, UOI, IOU, AM, PM, AD, BC, BCE, USA, UN, CF, ROFL, LOL, ETC
Property of The Thread
"Nobody knows the age of the human race, but everyone agrees it's old enough to know better." - Anon