feat: add nulls_equal to join - #3817
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Closes narwhals-dev#3615. Adds a nulls_equal parameter to DataFrame.join and LazyFrame.join, defaulting to False (current behaviour). With nulls_equal=True a null key matches another null key, matching polars. Each backend uses a different mechanism: polars passes it through (join_nulls before 1.24); pandas and dask skip the key dropna since their merge already matches nulls; duckdb uses an IS NOT DISTINCT FROM predicate; spark-like uses eqNullSafe; ibis uses identical_to; pyarrow joins on a null-safe encoding since its join drops null keys.
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Description
Adds a
nulls_equalparameter toDataFrame.joinandLazyFrame.join, matching polars.It defaults to
False(null keys never match, the current behaviour); withnulls_equal=Truea null key matches another null key.
Each backend needs a different mechanism, since their default null handling differs:
nulls_equal, orjoin_nullsbefore 1.24).mergealready matches nulls, so the existingdropnaon thekeys is skipped when
nulls_equal=True.IS NOT DISTINCT FROM((a = b) OR (a IS NULL AND b IS NULL)).Column.eqNullSafe.Column.identical_to.is_nullflag plus the value cast to string with null filled). The flag keeps a real""distinct from a null, so it is collision-free.nulls_equalapplies to every strategy exceptcross(which has no keys).What type of PR is this? (check all applicable)
Related issues
nulls_equalparameter tojoin#3615AI assistance
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