How to get to compare one column between two sheets to tell info in one sheet that is not in the other in Numbers
I think one way you can do this is through a formula using if(), iferror(), and vlookup() functions and also, if you wish, the Filter feature in Numbers.

Assuming you have the names of this year's clients in a table called "Clients 2020" and those of last year in "Clients 2019", you can identify the names of those that are yet to come this year with the formula shown in the picture below:



Here is a text version of the same formula if you wish to copy &amp; paste it from here:

= IF(IFERROR(VLOOKUP(B2,Clients 2020::B$2:B$21,1,0),0)=0,B2,0)


This formula lookups the name of the client in the cell on its left within the specified cells in the table called "Clients 2020". If the name is there, it returns "0". If not, it returns the name of the client. You may have to modify the formula a bit depending on the name of the table which has this year's clients and the exact cell addresses.

If you also wish to show only the yet-to-come (2019 only) clients as a list, you can then use Numbers' Filter feature. You can access this feature through the Organize option on the right in Numbers' toolbar at the top as shown in the picture below:



The filter shown in the picture above "filters" out any rows in the table "Clients 2019" whose "2019 only" column is equal to 0. 

If you would like to copy part or all of the filtered table elsewhere for further processing for any reason, select all the names (and, for example, the No column or any additional column you may have if you like) and press Command ⌘+C. Then select a cell in a new table (in a new sheet or file in Numbers if you also like) and then press Shift ⇧+Command ⌘+V (keyboard shortcut for "Paste Formula Results" menu option). These commands, when applied to a filtered table, will only copy &amp; paste the cells you selected from the filtered table.

Hope these help!