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Map Transform
Use map to select and reshape columns. Keep what you need, drop the rest.
Select Columns
$ Select Columns
Pick the columns you want. Everything else is gone.
Rename and Compute
$ Rename and Compute
Alias columns or create new ones from expressions. The result only contains what you specify.
Constants
$ Constants
Map can produce constant values across every row.
Map replaces the row shape — only mapped columns survive. Extend adds columns to the existing shape. Use map when you want to control exactly what comes out. Use extend when you want to add without losing anything.