What is the difference between UNION and UNION ALL?

Question

What is the difference between UNION and UNION ALL?

Answer 1

I add an example,

UNION, it is merging with distinct --> slower, because it need comparing (In Oracle SQL developer, choose query, press F10 to see cost analysis).

UNION ALL, it is merging without distinct --> faster.

SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual
UNION
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual;

and

SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual
UNION ALL
SELECT to_date(sysdate, 'yyyy-mm-dd') FROM dual;