Compare Two Columns of Numbers
Create a simple spreadsheet similar to the one below or use an existing copy of one of your sheets.
Orders sheet
Orders Processed Sheet
We now want to show on the first sheet what outstanding orders we have. Create a new heading on the Orders sheet called Confirmed Delivery.
In C2 use the following formula
=IF(COUNTIF('Orders Processed'!$A$2:$A$20,A2)>0,"Yes","")
This formula will count the occurrences of the Order Number from the Orders Sheet in the Orders Processed sheet and if it finds a match will show Yes or no match will show nothing. When you have entered the formula move back to that cell and you will see a small square on the bottom right of the cell 
. Double click on that square and it will automatically fill down the rest of the column.
This formula can be used in a variety of ways and makes it simpler to visually see any outstanding situations.