Replacing missings based on values in two DataFrame columns

General Usage
2

This should do it

julia> using DataFramesMeta, Random;

julia> begin
           x=100:111
           y=repeat(1:3,inner=4)
           z=vcat([shuffle([i,missing,missing,missing]) for i in 10:12]...)
           df = DataFrame(x=x, y=y, z=z)
       end
12×3 DataFrame
 Row │ x      y      z
     │ Int64  Int64  Int64?
─────┼───────────────────────
   1 │   100      1  missing
   2 │   101      1  missing
   3 │   102      1  missing
   4 │   103      1       10
   5 │   104      2  missing
   6 │   105      2  missing
   7 │   106      2  missing
   8 │   107      2       11
   9 │   108      3       12
  10 │   109      3  missing
  11 │   110      3  missing
  12 │   111      3  missing

julia> @chain df begin
           groupby(:y)
           @transform :z = first(skipmissing(:z))
       end
12×3 DataFrame
 Row │ x      y      z
     │ Int64  Int64  Int64
─────┼─────────────────────
   1 │   100      1     10
   2 │   101      1     10
   3 │   102      1     10
   4 │   103      1     10
   5 │   104      2     11
   6 │   105      2     11
   7 │   106      2     11
   8 │   107      2     11
   9 │   108      3     12
  10 │   109      3     12
  11 │   110      3     12
  12 │   111      3     12
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