Precedence Control in Table Operations

Control the execution order of UNION, EXCEPT, and INTERSECT operations. Learn how to use parentheses to define explicit operation priority.

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}]

Description

Using parentheses in SQL allows 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.

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 |
+------+

_{This page is licensed: CC BY-SA / Gnu FDL}