Syntax

• UNION DISTINCT – dedups after combining the SELECTs
• UNION ALL – non dedup (faster)
• UNION – the default is DISTINCT
• SELECT … UNION SELECT … – is OK, but ambiguous on ORDER BY
• ( SELECT … ) UNION ( SELECT … ) ORDER BY … – resolves the ambiguity

Remarks

UNION does not use multiple CPUs.

UNION always* involves a temp table to collect the results. *As of 5.7.3 / MariaDB 10.1, some forms of UNION deliver the results without using a tmp table (hence, faster).