ENH: Support Left Semi Joins

Is your feature request related to a problem?

I wish I could use the Pandas DataFrame.join function to perform left semi joins: selecting each row from the left DataFrame that has at least one matching row in the right DataFrame.

Describe the solution you’d like

I would like to be able to pass leftsemi to the how parameter in the DataDrame.join function to perform this type of join.

Additional context

For example: in SQL this might look like

SELECT *
FROM table1
WHERE EXISTS
(SELECT 1
FROM   table2
WHERE  table1.id = table2.id)

[debnathshoham]

Merge can happen using on, left_on & right_on, right_index & left_index, left_index & right_on and right_index & left_on. If none of these are provided, it tries on the common columns.

Yes, actually you have just considered 2 cases from the 5 I mentioned (you will get a clear picture, if you play around with all these existing merging procedures).

The idea is really simple, in leftsemi, check for the keys in left (can be either index(s) or column(s)) isin the keys in the right (again, could be either index(s) or column(s)). And return those keys from left.

You can check this example from stack. Hope this will clarify.

[debnathshoham]

Great! To answer the questions above:

1. Correct
2. Merge can happen using on, left_on & right_on, right_index & left_index, left_index & right_on and right_index & left_on. If none of these are provided, it tries on the common columns.
3. AFAIU semi join should only returns rows from left that has matching rows in right. It won’t add columns from right.

Hope that helps.