UNION removes duplicates (and pays a sort/hash); UNION ALL keeps them — correctness and cost both hinge on which.

UNION removes duplicate rows across the two inputs and pays a sort or hash to do it; UNION ALL keeps every row and skips that work. When each input row is a real event, dropping duplicates undercounts.

SELECT
  email
FROM
  promo
UNION SELECT
  email
FROM
  newsletter
ORDER BY
  email;

SELECT
  email
FROM
  promo
UNION ALL
SELECT
  email
FROM
  newsletter
ORDER BY
  email;

The 'total emails to send' count reads 3, not 4 — UNION silently merged the address that was on both lists.

Use UNION ALL when the rows are distinct events you want to keep; reach for UNION only when you actually want the duplicates removed.

SELECT ... UNION ALL SELECT ...