Equivalent of a VLOOKUP in Tableau?
There is a small problem in your calculation. You are creating a new column in existing table, perhaps, instead of joining two tables. Let me show this through sample superstore dataset with tableau. I have created a new excel table with few product IDs and designation.
Step-1: For sake of understanding products where name start with A are desig A and products starting with B are B and everything else is not included in table (your C).
Now you have successfully mapped your both tables just like
vlookup in excel does. Null values you can map to C either through intermediate calculation or directly.
Hope this helps. Else please upload your sheet.