Question

I am confused where should I start to design a star schema.

for example
I have tables in database as follows:

Branch(branchNo, bStreetAddress, bCity)
LoanManager(empNo, empName, phone, branchNo)
Customer(custNo, custName, profession, streetAddress, city, state)
Account(accNo, accType, balance, accDate, custNo)
LoanContract(contractNo, loanType, amount, loanDate, empNo, custNo)

I want to design a data-warehouse to analysis the loads
such as :

1. The total amount of loans in 2008.
2. For the type of loans with more than 10 loan contracts, the type of loan and the number of contracts

when creating a star schema, what where should I start?

For what I understanding, all the star schemas must have a center, and the center fact table, contains “Measures” and “Relations to other fact tables”.

So, is it that, when designing the star schema, we always start from the center,
confirm what are the measure first? and then choose proper relation to another fact table?

But I still have another question, what should we choose to be Measures?
When choosing measures, what question should I ask myself?

Answer 1

The design of a star schema is always driven by the client’s business needs. What are the questions asked? How fine-grained should the answers be?

In you example, interesting questions might be “Number of Contracts by Branch or LoanManager” or “Managed sum of Loans by Branch or LoanManager”. In this case, Branch and LoanManager would become your dimensions while Count(LoanContract) and Sum(LoanContract.amount) would be your measures. A common additional dimension is time, usually week or quarter.

The schema for answering those questions could look like this:

DimBranch ( branchNo )
DimLoanManager ( empNo )
DimQuarter ( year, qNo )  -- qNo in (1,2,3,4)
DimWeek ( year, weekNo )  -- weekNo in (0..53), depending on business rules

Measures ( branchNo, empNo, year, qNo, weekNo, numContracts, sumLoans )

For the business questions you already posed in your question, the dimensions and measures would be such:

1. dimension: year, measure: Sum(LoanContract.amount)
2. dimension: loanType, measure: Count(LoanContract)

Putting those two into the same star schema doesn’t make much sense, since they neither share dimensions or measures.