How can i define variable for default values in MariaDB structure? - mysql

Question

I need to change a lot default values of the DB structure for new entries in MULTIPLE tables to have this new default value in DB I tried to use something like this:
SET #money_rename_def := 20*10000;
SET #money_gender_def := 40*10000;
ALTER TABLE `money` CHANGE `money_rename` `money_rename` int(10) UNSIGNED NOT NULL DEFAULT '#money_rename_def',
CHANGE `money_gender` `money_gender` int(10) UNSIGNED NOT NULL DEFAULT '#money_gender_def';
But the SET prefix does not fork for alter table command. Is there any way how to do this to use pre-defined value so I can only change it once in SET or simillar definition?
I tried to search documentation but maybe just missed it?

Answer 1

You can use dynamic query like this:
DROP PROCEDURE IF EXISTS `tableDefaultSetter`$$
CREATE PROCEDURE `tableDefaultSetter`()
BEGIN
SET #default1 = 20;
SET #query = concat('ALTER TABLE `ttestt` CHANGE `val` `val` int NOT NULL DEFAULT ', #default1);
PREPARE stmt FROM #query;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END$$
Sample Data:
DROP TABLE IF EXISTS ttestt;
CREATE TABLE ttestt
(
id INT,
val INT(10) DEFAULT 10
);
INSERT INTO ttestt (id)
VALUES (1);
CALL tableDefaultSetter();
INSERT INTO ttestt (id)
VALUES (1);
SELECT *
FROM ttestt;
Result:
1,10
1,20
So the first item had 10 as default value and second item has been changed to 20. You see that it works.
For multiple values, you cannot put multiple queries inside one statement:
Doc
The text must represent a single statement, not multiple statements.
So you can create another procedure for convenience:
DELIMITER $$
DROP PROCEDURE IF EXISTS `exec_query`$$
CREATE PROCEDURE `exec_query`(queryStr TEXT)
BEGIN
SET #query = queryStr;
PREPARE stmt FROM #query;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END$$
DROP PROCEDURE IF EXISTS `tableDefaultSetter`$$
CREATE PROCEDURE `tableDefaultSetter`()
BEGIN
SET #default1 = 20;
SET #default2 = 30;
SET #default3 = 40;
SET #default4 = 50;
CALL exec_query(concat('ALTER TABLE `ttestt` CHANGE `val` `val` int NOT NULL DEFAULT ', #default1));
CALL exec_query(concat('ALTER TABLE `ttestt2` CHANGE `val` `val` int NOT NULL DEFAULT ', #default2));
CALL exec_query(concat('ALTER TABLE `ttestt3` CHANGE `val` `val` int NOT NULL DEFAULT ', #default3));
CALL exec_query(concat('ALTER TABLE `ttestt4` CHANGE `val` `val` int NOT NULL DEFAULT ', #default4));
END$$
DELIMITER ;
Use it like this:
CALL tableDefaultSetter();
DROP PROCEDURE `tableDefaultSetter`;
DROP PROCEDURE `exec_query`;