Assigning value to a string in Power Query

I have the following code:

let
Condition= Excel.CurrentWorkbook(){[Name="test_table"]}[Content],
field= Condition{0}[field_excel],  
str="One",
query = if field <> null
then str= "two" 
else str="three",
exec= Oracle.Database("TESTING",[Query=str])
in
exec

I want to depending the condition str have the value of two or three, but always stays with One