what frederico_fonseca wrote is almost correct.
but the f.num field is defined as varchar or char, then the value may contain a comma.
Example: num = 12,123: if it was saved as text.
If the data was imported from a CSV file, the value can also look like this:
num = 1.100.293 (one million one hundred thousand and two hundred and ninety-three)
First you have to import the values into the correct format, i.e. without separating thousands, which may have been set in excel.
Depending on the country setting, for example Germany, a decimal value with a comma is output 123,1456
This can be converted with TSQL Replace and converted from the varchar into decimal.
Hope to have helped you.