I think if you want to link a specific value from the database table to the context variable then you have to add a tOracleRow component and execute the following query:
"select a, b, c from table1 where a = '" + context.xyzVariable + "'";
Then you need to add a tJavaRow component and add the below code:
context.xyzVariable = input_row.myColumn;
I think this will work fine to give you the desired result.