merge() for SQL-style joins in Data Analysis Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the merge() function do in pandas?

The merge() function combines two DataFrames based on common columns or indices, similar to SQL joins.

beginner

Name the four main types of joins you can perform with merge().

The four main join types are: inner, left, right, and outer joins.

beginner

What is an inner join in merge()?

An inner join returns only rows with keys that appear in both DataFrames.

intermediate

How do you specify the columns to join on in merge()?

Use the on parameter to specify the column name(s) to join on. You can also use left_on and right_on for different column names.

beginner

What happens if you perform a merge() without specifying the how parameter?

By default, merge() performs an inner join, returning only matching rows from both DataFrames.

Which how parameter value returns all rows from the left DataFrame and matching rows from the right?

A right

B left

C inner

D outer

The 'left' join returns all rows from the left DataFrame and matching rows from the right.

If you want to join on columns with different names in each DataFrame, which parameters do you use?

A on

B how

C index

D left_on and right_on

Use 'left_on' and 'right_on' to specify different column names to join on.

What type of join does merge() perform by default?

A inner

B left

C right

D outer

By default, 'merge()' performs an inner join.

Which join type returns all rows from both DataFrames, filling missing values with NaN?

A outer

B left

C right

D inner

An outer join returns all rows from both DataFrames, filling missing matches with NaN.

What parameter would you use to join DataFrames based on their index?

A on

B how

C left_index and right_index

D sort

Use 'left_index=True' and 'right_index=True' to join on indices.

Explain how you would use merge() to combine two DataFrames with a common column named 'id'.

Think about the simplest join on a shared column.

Describe the difference between an inner join and an outer join using merge().

Consider what happens to rows without matches.