How to use prepared statements within MySQL stored procedures?

Question

I'm using mysql and I need to somehow use the column curid returned by the prepared statement in the later query. I use prepared statements because as I've read, it is the only way to pass a variable to the LIMIT clause. I have this stored procedure here:

DROP PROCEDURE IF EXISTS fixbalance;
CREATE PROCEDURE fixbalance (userid INT)
  BEGIN
  DECLARE i INT DEFAULT 0;
  DECLARE balance INT DEFAULT 0;
  DECLARE idcnt INT;

  SET idcnt = (SELECT COALESCE(COUNT(id), 0) 
               FROM coupon_operations 
               WHERE user_id = userid);
  IF idcnt <> 0 THEN
    WHILE i <= idcnt DO
      BEGIN
        SET @iter = i;
        SET @user_id = userid; 
        SET @sql = CONCAT('SELECT id AS curid 
                           FROM coupon_operations 
                           WHERE user_id = ? 
                           ORDER BY id ASC 
                           LIMIT ?, 1');
        PREPARE stmt FROM @sql;
        EXECUTE stmt USING @user_id, @iter;
        DEALLOCATE PREPARE stmt;
        SET balance = balance + (SELECT points 
                                 FROM coupon_operations 
                                 WHERE user_id = @user_id 
                                 AND id = @curid);
        UPDATE coupon_operations SET balance = balance;
        SET i = i + 1;
      END;
    END WHILE;
  END IF;
END;
|

This does not work - I'm not sure how to pass the curid.

score 11 · Answer 1 · answered Mar 28 '11 at 08:15

11

The solution was to SET the variable in the prepared statement itself as in:

SET @sql = CONCAT('SET @curid = SELECT id
                                FROM coupon_operations 
                                WHERE user_id = ? 
                                ORDER BY id ASC 
                                LIMIT ?, 1');

answered Mar 28 '11 at 08:15

donk

525
2
6
13

1

Whats's the point of `CONCAT` in the code above? – Pacerier Apr 02 '15 at 11:25
@Pacerier You don't need to use `CONCAT` unless you pass the table name or something as a parameter. ex) `CONCAT('select * from ', the_table_name, ' where id>100');` – Deckard Jun 14 '17 at 07:39

score 10 · Answer 2 · answered Mar 28 '11 at 13:25

I'm glad you found your answer. Another solution would be to use the SELECT...INTO syntax:

SET @sql = CONCAT('SELECT id INTO @curid FROM coupon_operations 
                   WHERE user_id = ? 
                   ORDER BY id ASC 
                   LIMIT ?, 1');

score 0 · Answer 3 · answered May 07 '16 at 07:35

Please tried this. to solve Concatenate, PREPARE and EXECUTE statements as bellow..

CREATE DEFINER=`products`@`localhost` PROCEDURE `generateMeritList`(
   IN `mastercategory_id` INT(11), 
   IN `masterschools_id` INT(11)
 )
 NO SQL
 begin

  declare total int default 0;
  declare conditions varchar(255) default ''; 
  declare finalQuery varchar(60000) default '';

if mastercategory_id > 0 THEN
    SET conditions =  CONCAT(' and app.masterschools_id = ', mastercategory_id);
end if;

SET @finalQuery = CONCAT(
 "SELECT * FROM applications app INNER JOIN masterschools school ON school.id = app.masterschools_id
WHERE 
 (app.status = 'active'", conditions, " LIMIT ",total); 

 PREPARE stmt FROM @finalQuery;
 EXECUTE stmt;
 DEALLOCATE PREPARE stmt;

end

This particular example is fine but I'd discourage using it as a general pattern due to **risk of SQL injection**. If the input was a string - such as VARCHAR - instead of INT, that'd potentially allow an attacker to inject SQL code. A properly-created prepared statement is safer than concatenating strings with external input. — Juanal, Sep 27 '19 at 07:33

How to use prepared statements within MySQL stored procedures?

3 Answers3