Syntax:
query_expression_body UNION [ALL | DISTINCT] query_block
    [UNION [ALL | DISTINCT] query_expression_body]
    [...]

query_expression_body:
    See 

UNION combines the result from multiple query blocks into a single
result set. This example uses SELECT statements:

mysql> SELECT 1, 2;
+---+---+
| 1 | 2 |
+---+---+
| 1 | 2 |
+---+---+
mysql> SELECT 'a', 'b';
+---+---+
| a | b |
+---+---+
| a | b |
+---+---+
mysql> SELECT 1, 2 UNION SELECT 'a', 'b';
+---+---+
| 1 | 2 |
+---+---+
| 1 | 2 |
| a | b |
+---+---+

UNION Handing in MySQL 8.0 Compared to MySQL 5.7

In MySQL 8.0, the parser rules for SELECT and UNION were refactored to
be more consistent (the same SELECT syntax applies uniformly in each
such context) and reduce duplication. Compared to MySQL 5.7, several
user-visible effects resulted from this work, which may require
rewriting of certain statements:

o NATURAL JOIN permits an optional INNER keyword (NATURAL INNER JOIN),
  in compliance with standard SQL.

o Right-deep joins without parentheses are permitted (for example, ...
  JOIN ... JOIN ... ON ... ON), in compliance with standard SQL.

o STRAIGHT_JOIN now permits a USING clause, similar to other inner
  joins.

o The parser accepts parentheses around query expressions. For example,
  (SELECT ... UNION SELECT ...) is permitted. See also [HELP
  parenthesized query expressions].

o The parser better conforms to the documented permitted placement of
  the SQL_CACHE and SQL_NO_CACHE query modifiers.

o Left-hand nesting of unions, previously permitted only in subqueries,
  is now permitted in top-level statements. For example, this statement
  is now accepted as valid:

(SELECT 1 UNION SELECT 1) UNION SELECT 1;

o Locking clauses (FOR UPDATE, LOCK IN SHARE MODE) are allowed only in
  non-UNION queries. This means that parentheses must be used for
  SELECT statements containing locking clauses. This statement is no
  longer accepted as valid:

SELECT 1 FOR UPDATE UNION SELECT 1 FOR UPDATE;

  Instead, write the statement like this:

(SELECT 1 FOR UPDATE) UNION (SELECT 1 FOR UPDATE);

URL: https://dev.mysql.com/doc/refman/8.0/en/union.html

