Syntax:
parenthesized_query_expression:
    ( query_expression [order_by_clause] [limit_clause] )
      [order_by_clause]
      [limit_clause]
      [into_clause]

query_expression:
    query_block [UNION query_block [UNION query_block ...]]
      [order_by_clause]
      [limit_clause]
      [into_clause]

query_block:
    SELECT ...             (see )

order_by_clause:
    ORDER BY as for SELECT (see )

limit_clause:
    LIMIT as for SELECT    (see )

into_clause:
    INTO as for SELECT     (see )

MySQL 8.0.22 and higher supports parenthesized query expressions
according to the preceding syntax. At its simplest, a parenthesized
query expression contains a single SELECT and no following optional
clauses:

(SELECT 1);
(SELECT * FROM INFORMATION_SCHEMA.SCHEMATA WHERE SCHEMA_NAME = 'mysql');

A parenthesized query expression can also contain a UNION comprising
multiple SELECT statements, and end with any or all of the optional
clauses:

mysql> (SELECT 1 AS result UNION SELECT 2);
+--------+
| result |
+--------+
|      1 |
|      2 |
+--------+
mysql> (SELECT 1 AS result UNION SELECT 2) LIMIT 1;
+--------+
| result |
+--------+
|      1 |
+--------+
mysql> (SELECT 1 AS result UNION SELECT 2) LIMIT 1 OFFSET 1;
+--------+
| result |
+--------+
|      2 |
+--------+
mysql> (SELECT 1 AS result UNION SELECT 2)
       ORDER BY result DESC LIMIT 1;
+--------+
| result |
+--------+
|      2 |
+--------+
mysql> (SELECT 1 AS result UNION SELECT 2)
       ORDER BY result DESC LIMIT 1 OFFSET 1;
+--------+
| result |
+--------+
|      1 |
+--------+
mysql> (SELECT 1 AS result UNION SELECT 3 UNION SELECT 2)
       ORDER BY result LIMIT 1 OFFSET 1 INTO @var;
mysql> SELECT @var;
+------+
| @var |
+------+
|    2 |
+------+

Parenthesized query expressions are also used as query expressions, so
a query expression, usually composed of query blocks, may also consist
of parenthesized query expressions:

(SELECT * FROM t1 ORDER BY a) UNION (SELECT * FROM t2 ORDER BY b) ORDER BY z;

Query blocks may have trailing ORDER BY and LIMIT clauses, which are
applied before the outer UNION and ORDER BY and LIMIT.

You cannot have a query block with a trailing ORDER BY or LIMIT,
without wrapping it in parentheses, but parentheses may be used for
enforcement in various ways:

o To enforce LIMIT on each query block:

(SELECT 1 LIMIT 1) UNION (SELECT 2 LIMIT 1);

o To enforce LIMIT on both query blocks and the entire query
  expression:

(SELECT 1 LIMIT 1) UNION (SELECT 2 LIMIT 1) LIMIT 1;

o To enforce LIMIT on the entire query expression (with no
  parentheses):

SELECT 1 UNION SELECT 2 LIMIT 1;

o Hybrid enforcement: LIMIT on the first query block and on the entire
  query expression:

(SELECT 1 LIMIT 1) UNION SELECT 2 LIMIT 1;

The syntax described in this section is subject to certain
restrictions:

o If ORDER BY occurs within a parenthesized query expression and also
  is applied in the outer query, the results are undefined and may
  change in a future version of MySQL. The same is true if LIMIT occurs
  within a parenthesized query expression and also is applied in the
  outer query.

o A trailing INTO clause for a query expression is not permitted if
  there is another INTO clause inside parentheses.

o Parenthesized query expressions do not permit multiple levels of
  ORDER BY or LIMIT operations. For example:

mysql> (SELECT 'a' UNION SELECT 'b' LIMIT 1) LIMIT 2;
ERROR 1235 (42000): This version of MySQL doesn't yet support 'parenthesized
query expression with more than one external level of ORDER/LIMIT operations'

URL: https://dev.mysql.com/doc/refman/8.0/en/parenthesized-query-expressions.html

