Split dataframe row into multiple rows

Specific Domains Data
2

Almost there:

julia> transform!(df, :B => (x -> ismissing(x) ? [missing] : string.(split(x, ";")), ) => :C)
4×3 DataFrame
 Row │ A      B        C
     │ Int64  String?  Vector
─────┼─────────────────────────────
   1 │     1  a; b     ["a", " b"]
   2 │     2  missing  [missing]
   3 │     3  b        ["b"]
   4 │     4  a; c     ["a", " c"]

julia> flatten(df, :C)
6×3 DataFrame
 Row │ A      B        C
     │ Int64  String?  String?
─────┼─────────────────────────
   1 │     1  a; b     a
   2 │     1  a; b      b
   3 │     2  missing  missing
   4 │     3  b        b
   5 │     4  a; c     a
   6 │     4  a; c      c
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