Joins and grouping transformations

A Polars join combines columns from one or more dataframes into a new dataframe; the joining strategy determines which rows are kept. The most common equi join matches rows by a key expression, and the default strategy is an inner join. Inner keeps matched rows from both sides; left outer keeps all left rows plus matching right rows, filling their right columns with null; right outer is a left outer join with the arguments swapped; full keeps all rows from either dataframe, filling non-matching sides with null; semi keeps left rows with a match on the right; anti keeps left rows without one. Polars supports non-equi joins, where the matching criterion is not equality, and asof joins, where rows match by key proximity. Scope: Polars. Reviewed October 4, 2026.