Precedence Control in Table Operations
MariaDB starting with 10.4.0
Beginning in MariaDB 10.4, you can control the ordering of execution on table operations using parentheses.
Syntax
( expression )
[ORDER BY [column[, column...]]]
[LIMIT {[offset,] row_count | row_count OFFSET offset}]
Contents
Description
Using parentheses in your SQL allows you to control the order of execution for SELECT statements and Table Value Constructor, including UNION, EXCEPT, and INTERSECT operations. MariaDB executes the parenthetical expression before the rest of the statement. You can then use ORDER BY and LIMIT clauses the further organize the result-set.
Note: In practice, the Optimizer may rearrange the exact order in which MariaDB executes different parts of the statement. When it calculates the result-set, however, it returns values as though the parenthetical expression were executed first.
Example
CREATE TABLE test.t1 (num INT); INSERT INTO test.t1 VALUES (1),(2),(3); (SELECT * FROM test.t1 UNION VALUES (10)) INTERSECT VALUES (1),(3),(10),(11); +------+ | num | +------+ | 1 | | 3 | | 10 | +------+ ((SELECT * FROM test.t1 UNION VALUES (10)) INTERSECT VALUES (1),(3),(10),(11)) ORDER BY 1 DESC; +------+ | num | +------+ | 10 | | 3 | | 1 | +------+
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