rankx dax

Nothing to do with RANKX(). Simplest table grouping in PQ does the trick.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("ddBBCsAgDATAv3huINmo1WPfUfz/N2ppU1PE25IdVvA8wxG2ICQEltijCpWeIfeZmZFD20xVa7oSLNk9BrzM8qRgDcrIk9Jviyk7lWJyancvlqWqX/PM6owwmvRHeaA8Gl1PCUVrQMILpa6yX63VuXYB", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Customer = _t, StartDate = _t, EndDate = _t, Department = _t]),

    Grouped = Table.AddIndexColumn(Table.Group(Source, "Department", {"Grp", each _}, GroupKind.Local),"SN",1,1),
    #"Expanded Grp" = Table.ExpandTableColumn(Grouped, "Grp", {"Customer", "StartDate", "EndDate"}, {"Customer", "StartDate", "EndDate"})
in
    #"Expanded Grp"