Stored procedure error when being called - mysql

Question

When I call my procedure, it return an error:
1064 - 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
How to fix it, do I have an error in my procedure code?
Here is my stored procedure :
DELIMITER $$
DROP PROCEDURE IF EXISTS `gamedb`.`ALIAS#SEARCH`$$
CREATE DEFINER=`root`#`localhost` PROCEDURE `ALIAS#SEARCH`(
in section_id varchar(255),
in category_id varchar(255),
in content_id varchar(255)
)
BEGIN
declare q varchar(4000);
set #q = 'SELECT * FROM tbl_alias WHERE ALIAS_ACTIVE_STATUS=1';
IF section_id IS NOT NULL THEN
set #q = concat(q,' AND ALIAS_SECTION_ID = ',section_id);
END IF;
IF category_id IS NOT NULL THEN
set #q = concat(q,' AND ALIAS_CATEGORY_ID = ',category_id);
END IF;
IF content_id IS NOT NULL THEN
set #q = concat(q,' AND ALIAS_CONTENT_ID = ',content_id);
END IF;
set #q= concat(q,' ORDER BY ALIAS_ID DESC');
prepare stmt from #q;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END$$
DELIMITER ;

Answer 1

There several problems
Since you can PREPARE only from user variable, you don't need a local variable q
You're missing # for #q variable when using CONCAT()
set #q = concat(q,' AND ALIAS_SECTION_ID = ',section_id);
^
Since your IN parameters defined as VARCHAR you better quote their values when you build the query
That being said your SP might look like this
DELIMITER $$
CREATE PROCEDURE `ALIAS#SEARCH`(
in section_id varchar(255),
in category_id varchar(255),
in content_id varchar(255)
)
BEGIN
SET #q = 'SELECT * FROM tbl_alias WHERE ALIAS_ACTIVE_STATUS = 1';
IF section_id IS NOT NULL THEN
SET #q = CONCAT(#q, ' AND ALIAS_SECTION_ID = ''', section_id, '''');
END IF;
IF category_id IS NOT NULL THEN
SET #q = CONCAT(#q, ' AND ALIAS_CATEGORY_ID = ''', category_id, '''');
END IF;
IF content_id IS NOT NULL THEN
SET #q = CONCAT(#q, ' AND ALIAS_CONTENT_ID = ''', content_id, '''');
END IF;
SET #q = CONCAT(#q, ' ORDER BY ALIAS_ID DESC');
PREPARE stmt FROM #q;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END$$
DELIMITER ;