Left join behavior in Pandas - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a left join do in pandas?

A left join keeps all rows from the left DataFrame and adds matching rows from the right DataFrame. If there is no match, it fills with NaN for the right DataFrame's columns.

beginner

Which pandas function is used to perform a left join?

The pandas function merge() is used with the argument how='left' to perform a left join.

beginner

What happens to rows in the right DataFrame that do not match any row in the left DataFrame during a left join?

Rows in the right DataFrame that do not match any row in the left DataFrame are ignored and do not appear in the result.

beginner

How does pandas represent missing data after a left join?

Pandas uses NaN (Not a Number) to represent missing data in columns from the right DataFrame when there is no matching row.

intermediate

If you want to keep all rows from both DataFrames, which join type should you use instead of left join?

You should use an outer join by setting how='outer' in merge() to keep all rows from both DataFrames.

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

A Keeps all rows from the left DataFrame and matches from the right

B Keeps only matching rows from both DataFrames

C Keeps all rows from the right DataFrame and matches from the left

D Keeps all rows from both DataFrames

A left join keeps all rows from the left DataFrame and adds matching rows from the right DataFrame.

What value fills columns from the right DataFrame when there is no match in a left join?

A 0

B NaN

C Empty string

D False

Pandas fills missing values with NaN when there is no matching row in the right DataFrame.

Which argument in pandas merge() specifies a left join?

A how='inner'

B how='outer'

C how='right'

D how='left'

Use how='left' to perform a left join in pandas merge.

If a row in the right DataFrame has no matching key in the left DataFrame, will it appear in the left join result?

A Yes, always

B Only if specified with an extra argument

C No, it will be excluded

D Only if the right DataFrame is smaller

Rows in the right DataFrame without a match in the left are excluded in a left join.

Which join type keeps all rows from both DataFrames?

A outer

B inner

C right

D left

An outer join keeps all rows from both DataFrames.

Explain how a left join works in pandas and what happens to unmatched rows from the right DataFrame.

Think about which DataFrame's rows always appear in the result.

Describe how to perform a left join in pandas and how missing data is represented after the join.

Focus on the function and the argument for left join.