Q:

How to subtract 2 columns with dtype = object within data frame to form a new column of the difference pandas

I have a merge data frame(mdf) which the 2 data frames are retrieved from SQL. I wish to create a new col within mdf which will be the subtraction of existing 2 columns.  

A:

I'm not sure what you mean by a "merge data frame," but here's a sketch of what you might be after. Please elaborate a little your question so it will be more useful to others.
df = pd.read_sql('select ....', some_sql_connection)

df['difference'] = df['some column name'] - df['another column name']

Also, referring to the title of your question where you mention dtype=object, data extracted from a SQL database sometimes defaults to the generic object datatype, even if it is actually numeric. (This is not ideal, and better handling of datatypes to and from SQL databases is being actively improved for a future release of pandas.)
For now, before manipulating your data, you might want to run df.convert_objects(convert_numeric=True) if you have all numerical data. See documentation.

