What is the difference between UNION and UNION ALL?

Question

What is the difference between UNION and UNION ALL?

Answer 1

You can avoid duplicates and still run much faster than UNION DISTINCT (which is actually same as UNION) by running query like this:

SELECT * FROM mytable WHERE a=X UNION ALL SELECT * FROM mytable WHERE b=Y AND a!=X

Notice the AND a!=X part. This is much faster then UNION.