看来你们两个都需要

SELECT total.*
FROM ( SELECT DATE_FORMAT(LEFT(CAST(reqdt AS UNSIGNED),8),'%Y-%m-%d') AS reqdt,
              COUNT(svce_domain) countsvce
       FROM bc_history
       GROUP BY reqdt
     UNION ALL
       SELECT DATE_FORMAT(LEFT(CAST(reqdt AS UNSIGNED),8),'%Y-%m-%d') AS reqdt,
              COUNT(svce_domain) countsvce
       FROM bc_history
       WHERE reqid ='MSYS'
       GROUP BY reqdt
     ) AS total

或在

SELECT DATE_FORMAT(LEFT(CAST(reqdt AS UNSIGNED),8),'%Y-%m-%d') AS reqdt,
       COUNT(svce_domain) total_count,
       SUM(reqid ='MSYS') msys_count
FROM bc_history
GROUP BY reqdt

根据修改后的问题更新
为什么不能使用

SELECT DATE_FORMAT(LEFT(CAST(reqdt AS UNSIGNED),8),'%Y-%m-%d') AS reqdt,
       COUNT(svce_domain) countsvce
FROM bc_history
WHERE reqid ='MSYS'
GROUP BY reqdt WITH ROLLUP;

?
列的总和将是最后一行，其中的值为空 reqdt 列（可以用一些文字替换，例如“total”）。
我想在springboot的mybatis中使用这个查询。但我的查询仅以别名发送数据。所以我需要结果作为别名。
我什么都不懂。但您可以将查询转换为子查询。

SELECT *
FROM (
SELECT DATE_FORMAT(LEFT(CAST(reqdt AS UNSIGNED),8),'%Y-%m-%d') AS reqdt,
       COUNT(svce_domain) countsvce
FROM bc_history
WHERE reqid ='MSYS'
GROUP BY reqdt WITH ROLLUP
) AS subquery
ORDER BY reqdt IS NULL; -- place total row last

现在按常规方式分配别名，就像这是一个表，而不是一个子查询。