Lets say you have a loaded table called 'Sales' with fields CustomerId, TransactionDate and TransactionAmount...
You could create another table that would sum the sales for each customer for the previous year like so...
, Sum(if(inyear(TransactionDate, Today(),-1),TransactionAmount)) AS sumOfSalesForLastYear
Group By CustomerId
Have a look at the inyear function in the QS help documentation. You will need to modify the above code for the specifics of your turnover calculation.
Hope that helps.
Does the sum() function compute correctly? If so, I am not sure how the result could be a string, but you could use the Num#() function?
Num#() converts a text string to a numerical value, in the number format set in the data load script or the operating system. Custom decimal and thousand separator symbols are optional parameters.