在/*fix vars */部分，如果monthnr〉12，则基于3个SET语句交换月份和日期值。但是，在过程结束时，SET visit_var = CONCAT(monthnr, '/', daynr, '/', yearnr);语句交换了字符串中的月和日的位置（与基表中的日/月/年格式相反），这意味着这些monthnr > 12没有任何变化。现在让我们删除这一部分来演示逻辑，但稍后您将需要它。

CREATE TABLE patient (
patno VARCHAR(3), -- patient number (3 digits),
visit VARCHAR(10) -- visit date (MM/DD/YYYY)
);

INSERT INTO patient VALUES 
 ('005','15/12/1999'),
 ('006','08/32/1998')
 ;
 
 DELIMITER //
drop procedure if exists change_date//
CREATE PROCEDURE change_date()
BEGIN
    DECLARE done INT DEFAULT FALSE;
    DECLARE patno_var VARCHAR(3);
    DECLARE visit_var VARCHAR(10);
    DECLARE daynr varchar(2);
    DECLARE monthnr varchar(2);
    DECLARE yearnr varchar(4);
    DECLARE temp varchar(2);
    DECLARE cur1 CURSOR FOR SELECT patno, visit FROM project33.patient;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

    OPEN cur1;
    read_loop: LOOP
        FETCH cur1 INTO patno_var, visit_var;
        IF done THEN
            LEAVE read_loop;
        END IF;

        /*split the visit col*/
        SET daynr = SUBSTRING(visit_var, 1, 2);
        SET monthnr = SUBSTRING(visit_var, LOCATE('/', visit_var, 2) + 1, 2);
        SET yearnr = SUBSTRING(visit_var, LOCATE('/', visit_var, 5) + 1, 4);

        /* let's remove this monthnr > 12 part for now but later on you'll find it necessary later on
        IF monthnr > 12 THEN 
            SET temp = monthnr; -- 32
            SET monthnr = daynr; -- 8
            SET daynr = temp; -- 32
            */

       IF monthnr IN (1, 3, 5, 7, 8, 10, 12) AND daynr > 31 THEN
            SET daynr = (daynr - 31);
            SET monthnr = (monthnr + 1);
  
        
       ELSEIF monthnr IN (4, 6, 9, 11) AND daynr > 30 THEN
            SET daynr = (daynr - 30);
            SET monthnr = (monthnr + 1);
        END IF;
    
  
    
        SET visit_var = CONCAT(monthnr, '/', daynr, '/', yearnr);
        
    
        /* update the visit column using the primary key */
        UPDATE patient SET visit = visit_var WHERE patno = patno_var;
    END LOOP;
    CLOSE cur1;
END //
DELIMITER ;

-- begin the test
select * from patient;
+-------+------------+
| patno | visit      |
+-------+------------+
| 005   | 15/12/1999 |
| 006   | 08/32/1998 |
+-------+------------+

 call change_date;

select * from patient;
+-------+------------+
| patno | visit      |
+-------+------------+
| 005   | 12/15/1999 |
| 006   | 32/08/1998 |
+-------+------------+

值已更改。然而，日期值“32/08/1998”几乎不合适。这就是为什么我说过程中的if monthnr > 12部分是必要的。您不需要交换month和day的值，但是您肯定需要进行条件检查，例如if monthnr >31 then ...
最后但并非最不重要的是，为了确保您的数据值符合条件，请考虑在更新表之前使用str_to_date函数进行格式检查。

SET visit_var = str_to_date(CONCAT(monthnr, '/', daynr, '/', yearnr),'%c/%e/%Y');