SQL query, find difference in 2 columns?

Question

MySQL

The table has 2 columns in each of which are numbers:
id1 | id2
100 876
135 657
129 768
200 200
489 489
How to find values ​​in the right column that are not in the left? (answer: 3 values: 876, 657, 768)

Answer 1

SELECT id2
FROM table t1
left join table t2 ON t1.id2=t2.id1 where
t2.id1 is null