你可以在下面试试-在我看来，你是想用 || 运营商mysql支持 concat() 功能不可用 || 操作人员

WITH A AS (SELECT OS.ACTUAL_DISPATCH_DATE,OT.ORDER_ID,OT.First_msg_date,datediff(OT.First_msg_date,OS.ACTUAL_DISPATCH_DATE) AS DATEDIFF,
CASE WHEN  datediff(OT.First_msg_date,OS.ACTUAL_DISPATCH_DATE)<=0 THEN 'OTIF' 
    WHEN  datediff(OT.First_msg_date,OS.ACTUAL_DISPATCH_DATE)=1 THEN 'OTIF+1'
    WHEN  datediff(OT.First_msg_date,OS.ACTUAL_DISPATCH_DATE)=2 THEN 'OTIF+2'
    ELSE  'OTIF+>2'
END as DATEDIFF_GRP
FROM order_shipment OS RIGHT JOIN 
(SELECT ORDER_ID,MIN((str_to_date(SUBSTRING(MESSAGE,INSTR(MESSAGE,'2'),10),'%Y-%m-%d'))) AS First_msg_date 
FROM ORDER_TIMELINE
GROUP BY ORDER_ID ) OT
ON (OS.ORDER_ID=OT.ORDER_ID)) 
SELECT DATEDIFF_GRP,concat(ROUND(COUNT(DISTINCT(ORDER_ID))*100/(SELECT COUNT(DISTINCT(A.ORDER_ID)) FROM A),2),'%') AS PERCENTAGE_ORDERS  
FROM A
GROUP BY DATEDIFF_GRP ;

mysql通常不使用 || 对于字符串连接，您必须启用一些非默认选项才能获得此结果。
现在的情况是mysql正在解释 || 作为逻辑 OR 接线员。要完全避免此问题，请考虑使用 CONCAT 功能。

SELECT
    DATEDIFF_GRP,
    CONCAT(100 * ROUND(COUNT(DISTINCT(ORDER_ID)) /
        (SELECT COUNT(DISTINCT(A.ORDER_ID)) FROM A), 2), '%') AS PERCENTAGE_ORDERS
FROM A;

您还可以通过启用 ANSI 但是正如下面的评论，很少有mysql用户在这种模式下使用数据库。