You want a fast, effienct way to find matches between two sets of order numbers.
Let's assume that one set of order numbers are in Sheet1 and this sheet is to be compared to those on Sheet2, and that row 1 in each sheet contains headers.
Enter this formula in Sheet1, Cell B2:
This tests for a match for each order number in Column A of Sheet1 with an order number anywhere in Column A of Sheet2. If a match is found, then the Sheet2 row number is displayed; if there is no match, then nothing is displayed. You could use conditional formatting to highlight matches; you could also use control totals such as
COUNTIF (to identify the number of matches).
Sheet1 | Sheet 2