Add right join

[mcrumiller]

Description

Join strategies currently are inner, left, outer, outer_coalesce, cross, semi, and anti. right joins are pretty standard but are missing from the list.

[sky-2002]

@mcrumiller I can add this, though it would be helpful if someone could point out where to add this, there are quite a lot of abstractions and function calling.

[owenprough-sift]

Can you provide an example of a scenario where a.join(b, how="right") is more ergonomic than writing b.join(a, how="left")? In a decade of heavy SQL use, I've never encountered a compelling use for a right join when left join exists.

[Wainberg]

There was a pull request a year and a half ago implementing right joins (issue #3934) but it wasn't merged. Right joins are super important to support! My PhD student calls the lack of right joins "my number 1 annoyance" with polars.

[bryngarrod-habitatenergy]

Can you provide an example of a scenario where a.join(b, how="right") is more ergonomic than writing b.join(a, how="left")? In a decade of heavy SQL use, I've never encountered a compelling use for a right join when left join exists.

The suffix argument is applied to columns from the right DataFrame, so you also have to deal with duplicate column names outside the join method.

[GuillaumePressiat]

For me left AND right joins are SQL standards and should be present in polars.
right joins are present in pandas, dplyr, data.table and all SQL engines.

Imagine a situation where you work on a small dataframe / table with many operations on it (ten or twenty lines of code).
Then you want to add this frame to a bigger dataframe, too late to think of it as a left join => inverse of a left join can be helpful to:

• simplify syntax
• keep the syntax from left to right in that case, (small table first, then join with big table)
• avoid refactoring code..

[alexander-beedie]

right joins are present in pandas, dplyr, data.table and all SQL engines.

I was thinking that SQLite doesn't have support for right joins, but double-checked and discovered that they actually (finally) added it back in 2022. Learned something new today 😆