merge tables

Assuming:

1) Sheet1, codes are in column A, we wish to add to this sheet
2) Sheet2, codes are in column A, we wish to extract from this sheet and add to sheet1
3) Sheet2, column M is empty and can be used to aid
4) Sheet2 row1 is titles or empty, data starts at row2

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1) In sheet2 M2 put this formula, then copy down

=ISNUMBER(MATCH(A2, Sheet1!A:A, 0))

Now you see a set if TRUE/FALSE answers for each row.

2) Highlight column M

3) Turn on the Data > Filter

4) Filter the column for FALSE

5) Highlight all the visible data in column A, copy and paste in on sheet1 below the current data

6) Turn off the sheet2 Filter and clear column M.