问题
Can anyone tell me what is chasm trap? Perhaps fan trap too as I'm not too clear. Also, please provide easy to understand examples (via Chen notations).
My understanding thus far: I understand that Fan trap is M:1:1:M, which suggests the paths between entities is ambiguous.
I understand that. For example, if M represents Student and the other M represents School then it'll be ambiguous because we don't know which student studies at which school (that's what I understood so far).
However, I cannot grasp what is chasm trap.
Also, how can I identify the traps and then fix it?
回答1:
In simple word, for both the cases (FAN & CHASM) it will produce more line(result sets) than actual. How to identify
- FAN -> 1-N-N means table relation from one -> many -> many
- CHASM -> N-1-N means one row table to two or more table many relation
- LOOP -> join all tables and when make loop like circle (In this case we will lose some rows absolutely)
Nothing to identify but when you create Universe than we have to keep our eyes open, if you see out of these situation while developing Universe than there will be a problem always. So rectify by applying aliases, context.
Once all problems solved at Universe level than we are good to go for reporting. By practice you will have excellent knowledge.
回答2:
Based on Conolly&Begg:
Fan trap occur in a situation when a model represents relationship between entity types however a path between certain entity occurrences is ambiguous.
Example: (Staff)-1:N-has-1:1-(Division)-1:1-operates-1:N-(Branch) in this model it may be impossible to determine the branch a staff belongs to, in the situation when staff belong to division having more than 1 branches.
Restructuring the model resolves trap (Division)-1:1-operates-1:N-(Branch)-1:1-has-1:N-(Staff)
Chasm trap occur when a model suggests relationship between entity types however a path between certain occurrences does not exist.
Example: (Branch)-1:1-has-1:N-(Staff)-0:1-oversees-0:N-(PropertyForRent)
Because Staff relationship to PropertyForRent is with optional participation (0:1) for staff the path for Branch to PropertyForRent may not exist. Solution to this would be direct relationship between Branch and PropertyForRent with mandatory participation.
回答3:
I fan trap occurs when three tables joins in a fashion where there realtion to each other is 1 to many way. means table A B and C are in join as .. table A links to table B in one to many and table B to table C relates again one to main way A-->B-->C.
来源：https://stackoverflow.com/questions/14328319/fan-trap-and-chasm-trap-database