Joins

Coral.Join matches rows by equality on one named key column. Use inner_join(left, right, on) for matches, left_join to keep every left row, and outer_join to include right-only rows too. Integer, float, and string keys work; bool keys fail for inner and left joins. A bool non-key column in either input causes any join to fail.

Joins traverse left rows in order. An outer join appends right-only rows after them. The right key column appears only once in the result. If a right non-key column has the same name as a left column, Coral adds the _right suffix.

module Coral.BookJoin
import Coral.Frame (FloatCol, StringCol, from_pairs, nrows, int_col_of_list)
import Coral.Join (left_join)
export (main)
def main() -> i64 = {
  left = from_pairs([("customer", StringCol(["a", "b", "a"])), ("qty", int_col_of_list([1i64, 2i64, 3i64]))])
  right = from_pairs([("customer", StringCol(["a", "c"])), ("score", FloatCol(to_tensor([10.0f32, 40.0f32])))])
  nrows(left_join(left, right, "customer"))
}

main returns 3. The unmatched b row has a NaN score. For either unmatched side of an outer join, a missing float value becomes NaN, a missing string becomes "", and a missing integer becomes 0 with a true missing-value mask. outer_join returns the key column as strings even when the input keys are integers or floats; inner and left joins keep the key's type.