Are you required to start with

dataset = Dataset[{{195, 2.3}, {198, 6.4}, {204, -3.1}}]

or do you have access to the raw data directly? E.g.

data = {{195, 2.3}, {198, 6.4}, {204, -3.1}}

For your second question, on adding units:

dataset = Dataset[{{195, 2.3}, {198, 6.4}, {204, -3.1}}]
Map[{Quantity[#[[1]],"Centimeters"],Quantity[#[[2]],"Volts"]}&,dataset//Normal]//Dataset

I have access to the raw data, and I would like to see an additional approach if I start with "data".

Thanks very much.

Regarding this instruction you wrote:

dataset[All,Key["c"]/*<|"ctotal"->Total,"clength"->Length|>] 

Can you show me how to write it in a "FullForm" notatation, With explicit RightComposition? I am not so familiar with this infix shortcut and don't fully understand which expression is the argument of which function.

Another question: You showed how to apply a function (Total and Length) on the values in each row.

Suppose I want to add 3 to each value, or multiply each value by 2, in column a, how to do it?

Another way, if you start with raw numbers:

list = {{195, 2.3}, {198, 6.4}, {204, -3.1}}

Note that both Thread[{"x", "y"} -> {195, 2.3}] and Thread[{"x", "y"} -> #] &[{195, 2.3}] return

{"x" -> 195, "y" -> 2.3}

Map the pure function Thread[{"x", "y"}->#]& on every sub-list:

Map[Thread[{"x", "y"} -> #] &, list]
(*{{"x" -> 195, "y" -> 2.3}, {"x" -> 198, "y" -> 6.4}, {"x" -> 204, "y" -> -3.1}}*)

and replace the head of each sub-list with that of Association:

Apply[Association, Map[Thread[{"x", "y"} -> #] &, list], {1}]
(*or*)
Association @@@ Map[Thread[{"x", "y"} -> #] &, list]
(* {<|"x" -> 195, "y" -> 2.3|>, <|"x" -> 198, "y" -> 6.4|>, <|"x" -> 204, "y" -> -3.1|>} *)

Only thing left is to make this a Dataset:

Dataset[%]

Written as a single instruction:

Association @@@ Map[Thread[{"x", "y"} -> #] &, list] // Dataset