Pandas DataFrame join() is an inbuilt function that is used to join or concatenate different DataFrames. The df.join() method join columns with other DataFrame either on an index or on a key column. Efficiently join multiple DataFrame objects by index at once by passing a list. We can either join the DataFrames vertically or side by side. By vertically, we mean joining the DataFrames column-wise, and side by side relates to indexing.

In simpler words, pd.join() can be defined as a way of joining standard fields of different DataFrames. The columns which contain common values and are used for joining are called join key.

To identify a joining key, we need to find the required data fields which are shared between the two data frames and the columns in that data frames, which are the same.

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