How DataFramesMeta @by works if you want to groupby two columns?

New to Julia
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3

You can use the @astable macro-flag for this.

julia> df = DataFrame(
                   a = repeat(1:4, outer = 2),
                   b = ["a", "b", "c", "d", "e", "f", "g", "h"],
                   c = [23,76,9,90,123,67,13,5]);

julia> @by df :a @astable begin 
           maxval, ind = findmax(:c)
           :c = maxval
           :b = :b[ind]
       end
4×3 DataFrame
 Row │ a      c      b      
     │ Int64  Int64  String 
─────┼──────────────────────
   1 │     1    123  e
   2 │     2     76  b
   3 │     3     13  g
   4 │     4     90  d
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