How to pass variable to UPDATE mysql query - mysql

Question

I wanted to replace a value (1 > -1) in a table with the following commands
UPDATE table_name
SET column_name = replace(column_name, '1', '-1');
However,I decided to learn how to use stored programs in Mysql, since the number of columns are large with their regularly formatted namesn ('i01', 'i02',...).
Below is my trial:
DELIMITER $$
DROP PROCEDURE IF EXISTS example$$
CREATE PROCEDURE example()
BEGIN
DECLARE p INT;
DECLARE str VARCHAR(20);
SET p = 1;
WHILE p < 100 DO
IF p <= 9 THEN SET str = CONCAT('i0', p);
ELSE SET str = CONCAT('i', p);
END IF;
UPDATE target_table
SET `str` = replace(str, '1', '-1');
SET p = p + 1;
END WHILE;
END$$
When I source the script, which was OK and call the function, there says ERROR 1054 (42S22): Unknown column 'str' in 'field list.'
How can I pass the variable, in this case #str, inside UPDATE query?
I searched online and found PREPARE could be an answer but could not figure out how to use that in my case.

Answer 1

Yes, you were on the right track with PREPARE.. the only issue is you have to concate the query string with the variable name outside of the string to access its contents.
SET #A = (SELECT CONCAT("UPDATE target_table SET `", #str, "` = replace(str, '1', '-1');"));
PREPARE qry FROM #A;
EXECUTE qry;
DEALLOCATE PREPARE qry;
if the replace you have is related to your variable #str (because you dont have the # sign there I'm not sure if its an actual column or the variable) then you need to change #A a little bit
SET #A = (SELECT CONCAT("UPDATE target_table SET `", #str, "` = replace(", #str, ", '1', '-1');"));
DEMO
with your DECLAREs this should be your final query
DELIMITER $$
DROP PROCEDURE IF EXISTS example$$
CREATE PROCEDURE example()
BEGIN
DECLARE p INT;
DECLARE str VARCHAR(20);
DECLARE update_qry VARCHAR(200);
SET p = 1;
WHILE p < 100 DO
-- set up column name
IF p <= 9
THEN SET str = CONCAT('i0', p);
ELSE SET str = CONCAT('i', p);
END IF;
-- set up query to execute
SET update_qry = CONCAT("UPDATE target_table SET `", str, "` = replace(", str, ", '1', '-1');")
-- prepare execute and deallocate query
PREPARE qry FROM update_qry;
EXECUTE qry;
DEALLOCATE PREPARE qry;
-- increment counter for next column name
SET p = p + 1;
END WHILE;
END$$