How to update a single column in target BODS

Hi,

I want to update a single column in target. Source contains two fields unique material and descriptions. i want to update the target last column ie description. Target does not contains primary key. material is duplicates i want to update all the records including duplicates.

Please advise me which method i should follow for updating single column.

source table

mat     desc

001     pen

002      book

target

mat     cat     desc

001     1     null    

001     2     null

002     1     null    

002     3     null

expected result:

mat     cat     desc

001     1     pen   

001     2     pen

002     1     book    

002     3     book

I tried with Table Comparison It is updating only one record not the entire material.