Re: Database design
Date: Tue, 21 Feb 2006 23:57:07 +0100
Message-ID: <43fb9aae$0$11076$e4fe514c_at_news.xs4all.nl>
Mark Johnson wrote:
> Alexandr Savinov wrote:
>
>>JOG schrieb: >> >>>Mark Johnson wrote: >>> >>>>x wrote: >>>> >>>>>Roy Hann wrote: >>>>> >>>>>>x wrote: >>>>> >>>>>Well, the slippery part is not that amusing after a while. >>>>> >>>>>>I am more inclined to read it as just the usual witless gaff of noticing >>>>>>that the bounding box of a printed representation of a table has length >>>>>>width and leaping to the conclusion that a table is therefore >>>>>>two-dimensional; planar: flat. >>>> >>>>Then I certainly stand to be corrected. I thought the relation was >>>>thought to be essentially an unordered set or list of entities, and >>>>nothing more.
>
>>>A tuple does not equate to an entity, in fact far from it.
>
>>So what is then an entity?
>
> As I understood it, the entity is each row in a relation, each entity
> consisting of various attributes, which alternately are termed,
> tuples. And each entity is supposed to be of 'like kind'.
> Unfortunately, based on previous messages, no one really seems to know
> what that is, or whether such 'like kinds' are even important.
People do tend to mix terminologies - but this is really horrible.
An entity is a thing of interest (old ISO definition).
Some prefer to talk about relations (or relational variables),
attributes and tuples.
Others about tables, columns and rows.
Though the concepts are different (relation <> table, attribute <>
column, row <> tuple), they are sufficiently similar to not prevent
people from communicating about databases.
Yet, neither tables nor relations map to entity types. One relation may have attributese from several entitiy types, and one entity may have data spread across several relations (or tables, if you prefer to use those terms). Received on Tue Feb 21 2006 - 23:57:07 CET