Add colum to dataframe with values based on various conditions

General Usage
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5

One more comment. If you are sure that right table does not have duplicates you can do:

julia> leftjoin!(df, df2, on=[:a, :c])
4×4 DataFrame
 Row │ a      b      c      value
     │ Int64  Int64  Int64  String?
─────┼──────────────────────────────
   1 │     1      5      9  result1
   2 │     2      6     10  missing
   3 │     3      7     11  result2
   4 │     1      8      9  result1

which maintains row order (it does in-place update so if you want a fresh data frame do leftjoin!(copy(df), df2, on=[:a, :c]))

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