**row_number():** Assigns 1,2,3,... per partition. No ties (unlike rank). Use for dedup (filter rn==1), pagination.
from pyspark.sql.window import Window
from pyspark.sql.functions import row_number
w = Window.partitionBy('dept').orderBy('salary')
df = df.withColumn('rn', row_number().over(w))
deduped = df.filter('rn == 1').drop('rn')
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