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What are the different types of Fatcs?
Explain with example only
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Answer Posted / hariharan v
There are four types of facts available in it.
1) Factless Fact
2) Coverage Fact
3) Incident Fact
4) Snapshot fact
Examples :
1) Factless fact:
Consider a school, where a single student may be taught by
many teachers and a single teacher may have many students.
To model this situation in dimensional model, one might
introduce a fact-less-fact table joining teacher and
student keys. Such a fact table will then be able to answer
queries like,
a. Who are the students taught by a specific teacher.
b. Which teacher teaches maximum students.
c. Which student has highest number of teachers.etc. etc.
2) Coverage fact:
A fact-less-fact table can only answer 'optimistic' queries
(positive query) but can not answer a negative query. Again
consider the illustration in the above example. A fact-less
fact containing the keys of tutors and students can not
answer a query like below,
a. Which teacher did not teach any student?
b. Which student was not taught by any teacher?
Why not? Because fact-less fact table only stores the
positive scenarios (like student being taught by a tutor)
but if there is a student who is not being taught by a
teacher, then that student's key does not appear in this
table, thereby reducing the coverage of the table
Coverage fact table attempts to answer this - often by
adding an extra flag column. Flag = 0 indicates a negative
condition and flag = 1 indicates a positive condition. To
understand this better, let's consider a class where there
are 100 students and 5 teachers. So coverage fact table
will ideally store 100 X 5 = 500 records (all combinations)
and if a certain teacher is not teaching a certain student,
the corresponding flag for that record will be 0.
3) Incident Fact:
A fact table stores some kind of measurements. Usually
these measurements are stored (or captured) against a
specific time and these measurements vary with respect to
time. This is example of an Incident Fact.
4) Snapshot Fact:
Take the example of the incident fact. Now it may happen
that the business might not able to capture all of its
measures always for every point in time. The above
unavailable measurements can be kept empty (Null) or can be
filled up with the last available measurements. This is the
example of snapshot fact.
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