Fill in Missing Gaps when Joining Data

Hi BugmanJ ,

I suggest you to copy M Query as below and paste it into Blank Query in Power Query Editor.

let
    Source = Table.NestedJoin(#"Table 2", {"Key"}, #"Table 1", {"Key"}, "Table 1", JoinKind.LeftOuter),
    #"Expanded Table 1" = Table.ExpandTableColumn(Source, "Table 1", {"Key", "Name", "Date"}, {"Table 1.Key", "Table 1.Name", "Table 1.Date"}),
    #"Reordered Columns" = Table.ReorderColumns(#"Expanded Table 1",{"Table 1.Key", "Table 1.Name", "Table 1.Date", "Key", "Payment"}),
    #"Added Conditional Column" = Table.AddColumn(#"Reordered Columns", "Table 1.Name1", each if [Table 1.Name] = null then "Unknown" else [Table 1.Name]),
    #"Reordered Columns1" = Table.ReorderColumns(#"Added Conditional Column",{"Table 1.Key", "Table 1.Name", "Table 1.Name1", "Table 1.Date", "Key", "Payment"}),
    #"Removed Columns" = Table.RemoveColumns(#"Reordered Columns1",{"Table 1.Name"}),
    #"Renamed Columns" = Table.RenameColumns(#"Removed Columns",{{"Table 1.Name1", "Table 1.Name"}, {"Key", "Table 2.Key"}, {"Payment", "Table 2.Payment"}})
in
    #"Renamed Columns"

New Table:

Best Regards,
Rico Zhou

If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.