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Re: The meaning pf "categories"? [Was: Re: Please advise regarding adesign of CIF dictionaries for material properties]
- To: "Discussion list of the IUCr Committee for the Maintenance of the CIFStandard (COMCIFS)" <comcifs@iucr.org>
- Subject: Re: The meaning pf "categories"? [Was: Re: Please advise regarding adesign of CIF dictionaries for material properties]
- From: Nick Spadaccini <nick@csse.uwa.edu.au>
- Date: Mon, 03 Oct 2011 11:04:19 +0800
- In-Reply-To: <4E88711D.6010205@ibt.lt>
On 2/10/11 10:11 PM, "Saulius Grazulis" <grazulis@ibt.lt> wrote: >> Again prop_ is a prefix, not something one would define as the category. > >> From the papers and the on-line information on CIF and DDL that are > accessible to me, I have got an impression that the term "category" is > used in two relatively unrelated meanings: > > a) DDL1 and increasingly DDLm use the "category" term to group related > items into categories, so that their attribute descriptions must not be > repeated. A category<->subcategory relation is then similar to a > "class<->subclass" relation on OO programming languages; DDL2 uses > "category group", "category" and "subcategory" terms for this. "parent" > and "child" in this context describe superclass and subclass relations; Originally DDL1 was a flat, no category structure, in much the same way as you have been using it. Over time (and influenced by the mm community) categories came in to it as a was of group data items which were related together. To be honest apart from a convenience I am not sure how much planning has gone in to DDL1 categories and how to use them. But they do provide a very simple abstract model. In DDLm we have developed a much more expansive syntax and semantics to the language. A hierarchy of categories and sub-categories in the full sense of building trees, encapsulation of data/definitions in to frames and nested frames, keys both primary and foreign, automatic natural joins on parent-child categories of a particular types etc. So there is a quite abstract model that over lays DDLm, which can be mapped in to any concrete model you want. A relation-model or a class/subclass OO model both naturally fall out of a dictionary in DDLm. > b) DDL2, by insisting on "category" <=> "single CIF loop_" equivalence > supposes that a "category" is essentially a "single relational database > table"; "parent" <-> "child" relations in this context are what the > RDBMS people would call "primary key" <-> "foreign key relations". The > terms "parent" and "child" might be unrelated to their usage in case a) > > Is that true - is my understanding correct? If not, may I please ask for > some clarifications where I am wrong? > > If b) is the case, dose it mean that DDL2 dictionary becomes essentially > a RDBMS scheme? DDL2 IS a purely relational model in the RDBMS sense. This has always been my difficulty with DDL2. There is no question it works well for mm people and in particular the PDB. I have no complaints about what DDL2 is for that community and the enormous contribution it makes to their function. However it is a concrete (back end) model for implementation (relational tables), that is constrained on the data representation at an exchange/archive level. If you are not going to adopt a relational model for implementation then DDL2 obfuscates what you need to represent. A purely abstract model at the DDL level is needed if you are going to require the seamless translation in to any number of possible concrete back-end models. cheers Nick -------------------------------- Associate Professor N. Spadaccini, PhD. Adjunct Research Fellow The University of Western Australia 35 Stirling Highway CRAWLEY, Perth, WA 6009 AUSTRALIA MBDP M002 CRICOS Provider Code: 00126G e: Nick.Spadaccini@uwa.edu.au
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