Inner join behavior in Pandas - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does an inner join do in pandas?

An inner join returns only the rows where the key exists in both DataFrames. It combines matching rows based on the join key.

beginner

Which pandas function is used to perform an inner join?

The pandas function merge() is used with the parameter how='inner' to perform an inner join.

intermediate

If DataFrame A has 5 rows and DataFrame B has 4 rows, how many rows can the inner join result have at most?

The inner join result can have at most the number of rows that have matching keys in both DataFrames, so it will be less than or equal to 4 (the smaller DataFrame's row count).

beginner

What happens to rows in pandas inner join if the key is missing in one DataFrame?

Rows with keys missing in either DataFrame are excluded from the result. Only rows with keys present in both DataFrames appear.

beginner

How can you specify the columns to join on in pandas inner join?

Use the on parameter in merge() to specify the column(s) to join on, e.g., pd.merge(df1, df2, on='key', how='inner').

What does how='inner' do in pandas merge()?

A Returns only rows with matching keys in both DataFrames

B Returns all rows from the left DataFrame

C Returns all rows from the right DataFrame

D Returns all rows from both DataFrames

Inner join returns only rows where the join key exists in both DataFrames.

If a key exists only in the left DataFrame, will it appear in the inner join result?

A Yes, always

B Only if specified with <code>left_on</code>

C Only if <code>how='outer'</code> is used

D No, never

Inner join excludes rows with keys missing in either DataFrame.

Which parameter specifies the join columns in pandas merge()?

A on

B columns

C join_cols

D keys

The on parameter specifies the column(s) to join on.

What is the default join type in pandas merge()?

A outer

B left

C inner

D right

The default join type for merge() is inner.

If DataFrame A has 3 matching keys with DataFrame B, how many rows will the inner join have?

A More than 3

B 3

C Less than 3

D Cannot be determined

Inner join returns rows only for matching keys, so the result will have 3 rows.

Explain how an inner join works in pandas and what happens to rows without matching keys.

Think about which rows appear in the result when keys match or don't match.

Describe how to perform an inner join on two pandas DataFrames using a specific column.

Focus on the function and parameters needed.