dia: in Chen's Notation, what are dot-bordered atts and what are multivalue atts for?

Question

I'm looking at , and I feel like I know what's going on until I see "PlayersIn" and "Foliage". Foliage is a double-line-bordered attribute because it is multivalued. What does that mean? How does it look in an actual database query? I don't know what the "PlayersIn" dotted-line bordering indicates. Can you please explain what it indicates, and when it is useful? What does it mean, what does a query look like, why is it important, etc.?

Answer 1

*Important note: the links contain java, but they are .ca websites and therefore likely secure.

Foliage is multi-valued, and I'm still unsure why anyone would want to do that. I read some documentation, here, on Chen Notation, that say: 'You may want to store degrees in a multi-value field, to show that someone has more than one degree, like a BA and an MS.' I don't understand why a foreign key for degree types isn't used with a dependency table, employeeID x degreeID? Maybe because it's less confusing to have only one table? As far as 'foliage' is concerned, I thought, 'Maybe there are many kinds of foliage, and it's unimportant to people. Maybe it's faster to use a multivalue field because joins are not necessary.'

As far as the yellow, dot-bordered attribute, it's a derived attribute. Derived attributes are attributes which can be obtained by parsing other attributes. Sometimes derived attributes are listed for human convenience, it appears. I'm not sure if it's only an issue of elegance, or if this can somehow speed the code up, though.