MySQL和BigQuery都允许with子句。也许你的数据在这些系统中是不同的？请提供MySQL版本详细信息。

请考虑对over使用Windows函数，而不是嵌套查询。在BiqQuery中测试：

With employee as 
(Select 19 age, true Attrition 
union all select 20,true
union all select 50,true)
SELECT  Agegroup, Attrition, Attrition / sum(Attrition) over ()
from (SELECT Agegroup, Count(*) as Attrition
   FROM(Select
    CASE
        WHEN age BETWEEN 0 AND 20 THEN '<20'
        WHEN age BETWEEN 20 AND 29 THEN '20-29'
        WHEN age BETWEEN 30 AND 39 THEN '30-40'
        WHEN age BETWEEN 40 AND 49 THEN '40-50'
        WHEN age BETWEEN 50 AND 100 THEN 'Greater than 50'
        ELSE 'not specified'
    END AS Agegroup
    FROM  employee
    Where Attrition = TRUE) AS T
    GROUP BY Agegroup) AS TT
ORDER BY Agegroup;