Sub rs_rm()

'This is a Microsoft Excel Visual Basic for Applications utility module for merging data.
'
'Data source set and destination data set both expected to be sorted by ID columns before running this module.
'Value from data source set overwrites value in destination set by ID match.
'The identifying keys here are not expected to necessarily be unique in each ID column, 
'and source data will be pasted to multiple destination rows where the identifying keys match.
' 
Dim d_id_r As Integer, d_id_c As Integer, d_rs_c As Integer
Dim s_id_r As Integer, s_id_c As Integer, s_rm_c As Integer
Dim d_id, s_id

d_id_r = 2  'data source set, identifying key, starting row
d_id_c = 1  'data source set, identifying key, column
d_rs_c = 3  'data source set, data column

d_id = Cells(d_id_r, d_id_c).Value

s_id_r = 2  'destination data set, identifying key, starting row
s_id_c = 5  'destination data set, identifying key, column
s_rm_c = 7  'destination data set, data column

s_id = Cells(s_id_r, s_id_c).Value

'loop until blank ID found, or 15-bit row limit approached
Do Until (d_id = "" Or s_id = "" Or (d_id_r > 30000) Or (s_id_r > 30000))
  Do Until (s_id = d_id) Or (s_id > d_id) Or s_id = ""
    s_id_r = s_id_r + 1
    s_id = Cells(s_id_r, s_id_c).Value
  Loop
  Do Until (s_id <> d_id)
    'Cells(s_id_r, s_rm_c).Value = Cells(d_id_r, d_rs_c).Value
    Cells(d_id_r, d_rs_c).Copy
    Cells(s_id_r, s_rm_c).Select
    ActiveSheet.Paste
    Cells(s_id_r, 6).Select  'this is just to conveniently see where the destination set
    ActiveSheet.Paste        'has been modified, by also pasting the copied data to column 6
    Cells(d_id_r, 2).Value = "'''"  'here I mark what source data rows were ID matched and used, in column 2
    s_id_r = s_id_r + 1
    s_id = Cells(s_id_r, s_id_c).Value
  Loop
  d_id_r = d_id_r + 1
  d_id = Cells(d_id_r, d_id_c).Value
Loop

End Sub