How to convert string from calc database to value in condition

I want to read a value from a mailmerge database (in this case an .ods file). Dependant on whether the value is >0 or <0 I want to substute different strings. The comparison in the condition field works only, if I compare to strings. I don’t find a way to convert a string from a field within an .ods file (where it is actually a value !) to to a value within “Write” so I can compare it to numerical 0. I could also not find any debugging help.