MySQL Stored Procedure fails with ERROR 1064 (42000) - mysql

Question

I have written a MySQL stored procedure that will add a new partition to an existing table:
DELIMITER //
DROP PROCEDURE IF EXISTS cr_par //
CREATE PROCEDURE cr_par (
IN p_table VARCHAR(256),
IN p_date DATE
) BEGIN
DECLARE stmt VARCHAR(1024);
DECLARE ddl VARCHAR(512);
DECLARE par_name VARCHAR(20) DEFAULT '';
DECLARE par_no INT DEFAULT 0;
DECLARE lt_value INT DEFAULT 0;
SET par_no = TO_DAYS(p_date) + 1;
SET par_name = CONCAT('p', par_no);
SET lt_value = par_no + 1;
SET ddl = CONCAT('ALTER TABLE ', p_table, ' ADD PARTITION (PARTITION ', par_name, ' VALUES LESS THAN (', lt_value, '))');
PREPARE stmt FROM #ddl;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SELECT ddl;
END //
DELIMITER ;
When I run the stored procedure I get this error:
mysql> CALL cr_par('test', '2021-09-13');
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'NULL' at line 1
If I comment out the PREPARE, EXECUTE and DEALLOCATE statements and re-run the stored procedure I get this, which is a valid DDL statement:
mysql> CALL cr_par('test', '2021-09-13');
+------------------------------------------------------------------------------+
| ddl |
+------------------------------------------------------------------------------+
| ALTER TABLE test ADD PARTITION (PARTITION p738412 VALUES LESS THAN (738413)) |
+------------------------------------------------------------------------------+
1 row in set (0.01 sec)
I've also tried these variations and all return the same error:
SET ddl = 'ALTER TABLE test ADD PARTITION (PARTITION p738412 VALUES LESS THAN (738413));';
PREPARE stmt FROM #ddl;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
and this...
SET ddl = CONCAT('ALTER TABLE ? ADD PARTITION (PARTITION ? VALUES LESS THAN (?))');
PREPARE stmt FROM #ddl;
EXECUTE stmt USING #p_table, #par_no, #lt_value;
DEALLOCATE PREPARE stmt;
I am using this version of MySQL:
Server version: 8.0.25-15 Percona Server (GPL), Release 15, Revision a558ec2
Does anyone have any ideas what could be causing this, I must be missing something simple?

Answer 1

SET ddl = CONCAT('ALTER TABLE ', p_table, ' ADD PARTITION (PARTITION ', par_name, ' VALUES LESS THAN (', lt_value, '))');
PREPARE stmt FROM #ddl;
Please be aware that ddl and #ddl are two different variables.
The variables you declare with the local variable DECLARE statement have a scope within the body of one stored routine. They are never spelled with a # sigil.
The user-defined variables with the # sigil have a scope of a MySQL session. You don't need to declare these kinds of variables. Just setting the variable to a value implicitly creates the variable.
You cannot SET ddl = ... and expect that string to be read from the #ddl variable. Nor vice-versa.
The PREPARE statement only supports preparing an SQL from a user-defined variable. Which means you must set the #ddl variable to your SQL statement:
SET #ddl = CONCAT('ALTER TABLE ', p_table, ' ADD PARTITION (PARTITION ', par_name, ' VALUES LESS THAN (', lt_value, '))');
Then you don't need to DECLARE ddl at all, since there's no use for that variable.