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Asked: 2026-05-11T19:42:03+00:00

Description: the query actually run have 4 results returned,as can be see from below,

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Description:
the query actually run have 4 results returned,as can be see from below,
what I did is just concate the items then return,
but unexpectedly,it’s null.

I think the code is self-explanatory:

DELIMITER |

DROP FUNCTION IF EXISTS get_idiscussion_ask|

CREATE FUNCTION get_idiscussion_ask(iask_id INT UNSIGNED) RETURNS TEXT DETERMINISTIC
BEGIN
  DECLARE done INT DEFAULT 0;
  DECLARE body varchar(600);
  DECLARE created DATETIME;
  DECLARE anonymous TINYINT(1);
  DECLARE screen_name varchar(64);
  DECLARE result TEXT;
  DECLARE cur1 CURSOR FOR SELECT body,created,anonymous,screen_name from idiscussion left join users on idiscussion.uid=users.id where idiscussion.iask_id=iask_id;
  DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET done = 1;

  SET result = '';
  OPEN cur1;
  REPEAT
    FETCH cur1 INTO body, created, anonymous, screen_name;
    SET result = CONCAT(result,'<comment><body><![CDATA[',body,']]></body>','<replier>',if(screen_name is not null and !anonymous,screen_name,''),'</replier>','<created>',created,'</created></comment>');
  UNTIL done END REPEAT;
  CLOSE cur1;

  RETURN result;
END |

DELIMITER ;

mysql> DELIMITER ;
mysql> select get_idiscussion_ask(1);
+------------------------+
| get_idiscussion_ask(1) |
+------------------------+
| NULL                   |
+------------------------+
1 row in set (0.01 sec)



mysql> SELECT body,created,anonymous,screen_name from idiscussion left join users on idiscussion.uid=users.id where idiscussion.iask_id=1;
+------+---------------------+-----------+-------------+
| body | created             | anonymous | screen_name |
+------+---------------------+-----------+-------------+
| haha | 2009-05-27 04:57:51 |         0 | NULL        |
| haha | 2009-05-27 04:57:52 |         0 | NULL        |
| haha | 2009-05-27 04:57:52 |         0 | NULL        |
| haha | 2009-05-27 04:57:53 |         0 | NULL        |
+------+---------------------+-----------+-------------+
4 rows in set (0.00 sec)

For those who don’t think the code is self-explanatory:

Why the function returns NULL?

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1 Answer

  1. Editorial Team

    Rename your variables and the input parameter, they’re ambiguous.

    This query:

    SELECT  body, created, anonymous, screen_name
FROM    idiscussion
LEFT JOIN
        users
ON      idiscussion.uid = users.id
WHERE   idiscussion.iask_id = iask_id

    returns your previously declared variables (which are NULL), not the table columns.

    Prepend the variable names and the input parameter name with an underscore.

    Also you make an extra assignment to result:

    FETCH cur1 INTO body, created, anonymous, screen_name;
SET result = CONCAT(result,'<comment><body><![CDATA[',body,']]></body>','<replier>',if(screen_name is not null and !anonymous,screen_name,''),'</replier>','<created>',created,'</created></comment>');

    The handler sets done after the FETCH fails, but the result gets assigned nevertheless.

    Change your handler to:

    DECLARE EXIT HANDLER FOR SQLSTATE '02000' RETURN result;

    Finally: in MySQL, this can be done with a single query. There is no need to do it with a function.

    SELECT  GROUP_CONCAT(CONCAT(result,'<comment><body><![CDATA[',body,']]></body>','<replier>',if(screen_name is not null and !anonymous,screen_name,''),'</replier>','<created>',created,'</created></comment>') SEPARATOR '')
FROM   idiscussion
LEFT JOIN
       users
ON     idiscussion.uid=users.id
WHERE  idiscussion.iask_id = @_iask_id
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