15.2.18 UNION Clause

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

query_expression_body:
    See Section 15.2.14, “Set Operations with UNION, INTERSECT, and EXCEPT”

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:

NATURAL JOIN permits an optional INNER keyword (NATURAL INNER JOIN), in compliance with standard SQL.
Right-deep joins without parentheses are permitted (for example, ... JOIN ... JOIN ... ON ... ON), in compliance with standard SQL.
STRAIGHT_JOIN now permits a USING clause, similar to other inner joins.
The parser accepts parentheses around query expressions. For example, (SELECT ... UNION SELECT ...) is permitted. See also Section 15.2.11, “Parenthesized Query Expressions”.
The parser better conforms to the documented permitted placement of the SQL_CACHE and SQL_NO_CACHE query modifiers.
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;
```
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);
```