SELECT COALESCE(Col1,Col2) AS TheValue FROM YourTableThe rule with COALESCE is straightforward; travel from left to right and pick the first non-null value you see. So in this case, if Col1 is not null, that is what you will get. If Col1 is NULL, you will get whatever is in Col2.
Now if you want to assign that to a variable, you need to pick one row from the table. Otherwise, what will get assigned is not predictable.
SELECT TOP(1) @var = COALESCE(Col1,Col2) AS FROM YourTable ORDER BY Col1,Col2