I have two columns. Column K has a bunch of values, Column L has some of these values. I want to filter everything that appears in Column L out of Column K. How would I do this?
I assume you want to get the values in K that are not appearing in L ? If so (and assuming your data starts on row 2), try:
=filter(K2:K, isna(match(K2:K, L2:L,0)))
If you only want to see unique values, then try:
=unique(filter(K2:K, isna(match(K2:K, L2:L,0))))
The possible solution to calculate intersection of Column K and Column L is:
=FILTER(UNIQUE(K2:K), (ARRAYFORMULA( COUNTIF(UNIQUE(L2:L), ARRAYFORMULA( UNIQUE(K2:K) ))))>0)
Please, see prepared example here: Columns Intersection