WITH
cte1 AS ( 
SELECT value, ROW_NUMBER() OVER (PARTITION BY value) rn
FROM a
),
cte2 AS ( 
SELECT value, ROW_NUMBER() OVER (PARTITION BY value) rn
FROM b
)
SELECT value
FROM cte1
WHERE NOT EXISTS ( SELECT NULL
                   FROM b
                   WHERE (a.value, a.rn) = (b.value, b.rn) )