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

Recall & Review

beginner

What is an outer join in database terms?

An outer join returns all rows from one or both tables, including rows that do not have matching values in the other table. Missing matches show as NULL.

beginner

What is the difference between left outer join and right outer join?

Left outer join returns all rows from the left table and matched rows from the right. Right outer join returns all rows from the right table and matched rows from the left.

beginner

In Python's pandas, which function is used to perform an outer join?

The pandas function merge() with the argument how='outer' performs an outer join.

intermediate

What does a full outer join return?

A full outer join returns all rows from both tables. Rows without matches in the other table have NULLs in the missing columns.

beginner

Why might you use an outer join instead of an inner join?

Use an outer join when you want to keep all data from one or both tables, even if there are no matching rows, to avoid losing information.

Which join returns all rows from the left table and matched rows from the right table?

A Left outer join

B Right outer join

C Inner join

D Cross join

Left outer join keeps all rows from the left table and matches from the right.

In pandas, how do you specify an outer join using the merge function?

A how='outer'

B how='left'

C how='inner'

D how='right'

Use how='outer' to perform an outer join in pandas merge.

What does a full outer join include that an inner join does not?

A Only rows from the left table

B Only matching rows

C All rows from both tables, including unmatched

D Only rows from the right table

Full outer join includes all rows from both tables, unmatched rows show NULLs.

If you want to keep all rows from the right table regardless of matches, which join do you use?

A Left outer join

B Right outer join

C Inner join

D Full outer join

Right outer join keeps all rows from the right table.

What value appears in columns from the other table when there is no match in an outer join?

A 0

B Empty string

C False

D NULL

Missing matches show as NULL in outer joins.

Explain what an outer join does and give an example of when you might use it.

Think about keeping all data from one or both tables even if no match exists.

Describe the difference between left outer join, right outer join, and full outer join.

Focus on which table's rows are fully kept in each join type.