IS [NOT] DISTINCT FROM is the simplest way of doing this.

On previous versions you can do (Fiddle)

• EXISTS (SELECT colA INTERSECT SELECT colB) for IS NOT DISTINCT
• EXISTS (SELECT colA EXCEPT SELECT colB) for IS DISTINCT

though the more verbose

• NOT EXISTS (SELECT colA INTERSECT SELECT colB)

sometimes gives a better execution plan for the IS DISTINCT case (example).

See Undocumented Query Plans: Equality Comparisons for some discussion on various alternative approaches for this.

Generally I’ve found execution plans are fine with the above approaches though occasionally I have found writing out the condition in full as

• WHERE ColA = ColB OR (ColA IS NULL AND ColB IS NULL) (for the NOT DISTINCT case)
• WHERE ColA <> ColB OR (ColA IS NULL AND ColB IS NOT NULL) OR (ColA IS NOT NULL AND ColB IS NULL) (for the DISTINCT case)

does give a better execution plan.

The INTERSECT/EXCEPT are very convenient when you need to do this type of comparison across multiple columns…

EXISTS (SELECT t1.colA, t1.colB, t1.colC 
        INTERSECT 
        SELECT t2.colA, t2.colB, t2.colC)

… but check the plans to see that they are being optimised efficiently in the context you are using them.

I didn’t get what you are asking here by the example. But heres a simpler version of your query

Select *, case when distinct_from_v1 = 1 then 0 else 1 end as not_distinct_from_v1
  from 
   (SELECT colA, colB, 
           CASE WHEN colA IS DISTINCT FROM colB THEN 1 ELSE 0 END AS distinct_from,
           case when Cola + colb is not null 
                then 
                case when cola=colb then 0 
                     else 1 
                end
           else
                case when concat(cola,colb) in (cola,colb)  then 1 else 0 end
      end  as distinct_from_v1
    FROM tab) t;

Working fiddle
https://dbfiddle.uk/aVKsWNvz

A simpler way is to use a NOT from the Equals way to write it. For example:

WITH tbl(colA, colB) AS (
    SELECT 1,2 UNION ALL
    SELECT NULL,1 UNION ALL
    SELECT 1,NULL UNION ALL
    SELECT NULL,NULL
) 
SELECT 
    colA, 
    colB,
    colA=colB OR (colA IS NULL AND colB IS NULL) eq,
    NOT( colA=colB OR (colA IS NULL AND colB IS NULL) ) neq
FROM
    tbl