Pandas populate new dataframe column based on matching columns in another dataframe

Question

I have a df which contains my main data which has one million rows. My main data also has 30 columns. Now I want to add another column

Answer 1

Consider the following dataframes df and df2

df = pd.DataFrame(dict(
        AUTHOR_NAME=list('AAABBCCCCDEEFGG'),
        title=      list('zyxwvutsrqponml')
    ))

df2 = pd.DataFrame(dict(
        AUTHOR_NAME=list('AABCCEGG'),
        title      =list('zwvtrpml'),
        CATEGORY   =list('11223344')
    ))

option 1
merge

df.merge(df2, how='left')

option 2
join

cols = ['AUTHOR_NAME', 'title']
df.join(df2.set_index(cols), on=cols)

---

both options yield