Iterate over rows in a stored procedure?

To iterate row-by-row over a query result set inside a stored procedure, use a CURSOR:

### Cursor Lifecycle Steps:

1. DECLARE: Define the cursor and query.
2. OPEN: Initialize the cursor result set.
3. FETCH: Retrieve the next row into local variables.
4. CLOSE: Free cursor memory resources.

DELIMITER //
CREATE PROCEDURE ProcessCustomers()
BEGIN
    DECLARE done INT DEFAULT FALSE;
    DECLARE cust_id INT;
    DECLARE cur CURSOR FOR SELECT id FROM Customers;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

    OPEN cur;
    read_loop: LOOP
        FETCH cur INTO cust_id;
        IF done THEN
            LEAVE read_loop;
        END IF;
        -- Process each customer ID here
    END LOOP;
    CLOSE cur;
END //
DELIMITER ;

*Performance Warning:* Databases are optimized for set-based operations. Cursors execute row-by-row ($O(n)$ procedural execution) and should only be used when set-based SQL logic is impossible.