combine dataframes in order of 1 column code example

Example 1: python: left join

new_df = df_1.merge(df_2, on='id', how='left', indicator=True)

Example 2: python - match two df on a variable with different name

pd.merge(df1, df2, left_on=  ['userid', 'column1'],
                   right_on= ['username', 'column1'], 
                   how = 'left')