Merge Two DataFrames in Python

3 min readApr 20, 2021

Pandas provide a single function, merge, as the entry point for all standard database join operations between DataFrame objects.

Syntex:

pd.merge(left, right, how=’inner’, on=None, left_on=None, right_on=None, left_index=False, right_index=False, sort=True)

Parameters details are as follows:

left − A DataFrame object.
right − Another DataFrame object.
on − Columns (names) to join on. Must be found in both the left and right DataFrame objects.
left_on − Columns from the left DataFrame to use as keys. Can either be column names or arrays with length equal to the length of the DataFrame.
right_on − Columns from the right DataFrame to use as keys. Can either be column names or arrays with length equal to the length of the DataFrame.
left_index − If True, use the index (row labels) from the left DataFrame as its join key(s). In the case of a DataFrame with a MultiIndex (hierarchical), the number of levels must match the number of join keys from the right DataFrame.
right_index − Same usage as left_index for the right DataFrame.
how − One of 'left', 'right', 'outer', 'inner'. Defaults to inner. Each method has been described below.
sort − Sort the result DataFrame by the join keys in lexicographical order. Defaults to True, setting to False will improve the performance substantially in many cases.

Code Example:

create 2 DataFrame to the demo merge operation

Merge 2 DataFrames on Key

Merge Two DataFrames on Multiple Keys

Here we merge based on id **and** subject_id

Merge Using ‘how’ Argument

The how argument to merge specifies how to determine which keys are to be included in the resulting table. If a key combination does not appear in either the left or the right tables, the values in the joined table will be NA.

Here is a summary of the how options and their SQL equivalent names −

Merge Method SQL Equivalent Description left LEFT OUTER JOIN Use keys from left object right RIGHT OUTER JOIN Use keys from right object outer FULL OUTER JOIN Use union of keys inner INNER JOIN Use intersection of keys.