Inner join in Data Analysis Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an inner join in database terms?

An inner join combines rows from two tables where the join condition is true, returning only matching rows from both tables.

beginner

In an inner join, what happens to rows that do not have matching values in both tables?

Rows without matching values in both tables are excluded from the result set.

beginner

How do you perform an inner join using pandas in Python?

Use pandas.merge(left_df, right_df, on='key_column', how='inner') to join two dataframes on a key column.

intermediate

Why is an inner join useful in data analysis?

It helps combine related data from different tables, showing only the rows where there is a match, which is useful for focused analysis.

intermediate

What is the difference between an inner join and a left join?

An inner join returns only matching rows from both tables, while a left join returns all rows from the left table and matching rows from the right table (or NaN if no match).

What does an inner join return?

A Only rows with matching keys in both tables

B All rows from the left table

C All rows from the right table

D All rows from both tables

An inner join returns only rows where the join condition matches in both tables.

Which pandas function is used to perform an inner join?

A pandas.concat()

B pandas.append()

C pandas.join() with how='outer'

D pandas.merge() with how='inner'

Use pandas.merge() with the argument how='inner' to perform an inner join.

If a row in the left table has no matching row in the right table, will it appear in an inner join result?

A Yes, always

B No, it will be excluded

C Only if specified with a condition

D Only if the right table is empty

Inner join excludes rows without matching keys in both tables.

Which SQL keyword is used to specify an inner join?

A INNER JOIN

B OUTER JOIN

C LEFT JOIN

D JOIN

INNER JOIN explicitly specifies an inner join in SQL.

What is the main purpose of using an inner join?

A To remove duplicates from a table

B To combine all rows from two tables

C To combine only matching rows from two tables

D To sort data in a table

Inner join combines only rows with matching keys from both tables.

Explain what an inner join does and give a simple example of when you might use it.

Think about how you combine two lists but keep only the common items.

Describe how to perform an inner join using pandas in Python and what parameters are important.

Remember the function that combines dataframes based on a key.