/// <inheritdoc cref="State{T}.Sum"/>
public override FrequenciesAndNumRows Sum(FrequenciesAndNumRows other)
{
IEnumerable <string> columns = Frequencies.Schema().Fields
.Select(field => field.Name)
.Where(field => field != AnalyzersExt.COUNT_COL);
IEnumerable <Column> projectionAfterMerge = columns
.Select(col =>
Coalesce(Col($"this.{col}"), Col($"other.{col}")).As(col))
.Append(
(AnalyzersExt.ZeroIfNull($"this.{AnalyzersExt.COUNT_COL}") +
AnalyzersExt.ZeroIfNull($"other.{AnalyzersExt.COUNT_COL}")).As(AnalyzersExt.COUNT_COL));
Column joinCondition = columns.Aggregate(NullSafeEq(columns.First()),
(previous, result) => previous.And(NullSafeEq(result)));
DataFrame frequenciesSum = Frequencies
.Alias("this")
.Join(other.Frequencies.Alias("other"), joinCondition, "outer")
.Select(projectionAfterMerge.ToArray());
return(new FrequenciesAndNumRows(frequenciesSum, NumRows + other.NumRows));
}