Create a Request with conditions for a column

Ah, ok. Try this:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Lc27DcAgDAXAVSLXFMGGfEoTZQG6CLH/GsF+Lq+6MYgrJdK300wLh6E/wOn4gMvRtgzeQXbKHhQwBwvIwQrKYlMFisH6+QM=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Nb = _t, Title = _t]),
    chgTypes = Table.TransformColumnTypes(Source,{{"Nb", Int64.Type}, {"Title", type text}}),

// Relevant steps from here:
    addTitleGroup = Table.AddColumn(chgTypes, "titleGroup", each Text.Select([Title], {"A".."Z"})),
    groupRows = Table.Group(addTitleGroup, {"titleGroup"}, {{"data", each _, type table [Nb=nullable number, Title=nullable text, titleGroup=text]}}),
    addCalcNewNb = Table.AddColumn(groupRows, "calcNewNb", each List.Min([data][Nb])),
    expandDataCol = Table.ExpandTableColumn(addCalcNewNb, "data", {"Title", "Nb"}, {"Title", "Nb"})
in
    expandDataCol

To get this output:

Pete