MySQL syntax error when using prepared statement with CONCAT - mysql

Question

I'm trying to use a prepared statement within a stored procedure. I'm getting the syntax error in the initial SET #idToUpdateQuery. I already know that the CONCAT without the variable assignment using := works correctly, as it's used elsewhere and works as intended. Essentially, I just need to be able to get the result of the prepared SELECT statement into the variable #resultId. The problematic code is as follows:
SET #distinctTableXIds = (SELECT COUNT(*) FROM tempTableX);
SET #i = 0;
WHILE #i < #distinctTableXIds DO
/*CONCAT IS NEEDED HERE TO PASS USER DEFINED #i*/
SET #idToUpdateQuery = CONCAT('SELECT #result := MAX(id)
FROM tableY
WHERE tableZId = (SELECT id FROM tempTableX ORDER BY id LIMIT ', #i, ', 1)');
PREPARE #IdToUpdateStmt FROM #IdToUpdateQuery;
EXECUTE #IdToUpdateStmt;
SET #resultId = SELECT #result;
UPDATE tableY
SET someBoolean = 1
WHERE id = #resultId;
SET #i = #i + 1;
END WHILE;

Answer 1

You don't use the # sigil for a statement name. It's not a user variable.
See example at https://dev.mysql.com/doc/refman/8.0/en/sql-prepared-statements.html
mysql> PREPARE stmt1 FROM 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
mysql> SET #a = 3;
mysql> SET #b = 4;
mysql> EXECUTE stmt1 USING #a, #b;
Besides that, I don't think you need a prepared statement for this task at all. I can think of at least two other solutions. Give me a few minutes and I'll write one up.
One solution using a cursor (see https://dev.mysql.com/doc/refman/8.0/en/cursors.html):
BEGIN
DECLARE done INT DEFAULT FALSE;
DECLARE max_id INT;
DECLARE cur1 CURSOR FOR SELECT MAX(id) FROM tableY JOIN tempTableX ON (tableY.tableZId = temptableX.id) GROUP BY tableZId;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN cur1;
read_loop: LOOP
FETCH cur1 INTO max_id
IF done THEN
LEAVE read_loop;
END IF;
UPDATE tableY SET someBoolean = 1 WHERE id = max_id;
END LOOP;
CLOSE cur2;
END
Another solution that does it all in one UPDATE statement:
UPDATE tableY AS y1
INNER JOIN tempTableX AS x ON (y1.tableZId = x.id)
LEFT OUTER JOIN tableY AS y2 ON (y1.tableZId = y2.tableZId AND y1.id < y2.id)
SET y1.someBoolean = 1
WHERE y2.id IS NULL;
The outer join trick is a way to find the row of y1 for which no row exists with the same tableZId and a greater id. In other words, it finds the largest id for each group of rows with a given tableZId.