Recode multiple columns in DataFrame as integer

General Usage
5

A fun little two liner without any additional packages:

julia> uvals = sort(unique([df.x; df.y]))
3-element Vector{Symbol}:
 :a
 :b
 :c

ulia> leftjoin(leftjoin(df, DataFrame(x = uvals, new_x = 1:length(uvals)), on = :x), DataFrame(y = uvals, new_y = 1:length(uvals)), on = :y)
5×4 DataFrame
 Row │ x       y       new_x   new_y  
     │ Symbol  Symbol  Int64?  Int64? 
─────┼────────────────────────────────
   1 │ a       b            1       2
   2 │ a       b            1       2
   3 │ b       c            2       3
   4 │ c       a            3       1
   5 │ c       c            3       3
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