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我有一个SELECT语句是从一个CASE WHEN THEN状态(或者可以使用多个IF语句)别名为'Length'计算的,我需要正确地将结果组合在一起.SELECT似乎正在工作,但该组将它们分组错误.这是我的发言:

SELECT CASE 
    WHEN DATEDIFF(o.EndDate, o.StartDate) < 30 THEN '<1 Month'
    WHEN DATEDIFF(o.EndDate, o.StartDate) < 90 THEN '1 - 2 Months'
    WHEN DATEDIFF(o.EndDate, o.StartDate) < 210 THEN '3 - 4 Months'
    ELSE '>4 Months' END AS 'Length', 
    COUNT(DISTINCT(person.ID)) AS 'COUNT'
FROM person
    INNER JOIN opportunity AS o
    INNER JOIN Organization AS org
    ON person.EntityID = o.id 
        AND O.OrganizationID = Org.ID
WHERE person.TitleID = 2
    AND o.bID = 1
GROUP BY 'Length'
ORDER BY 'Length' ASC;

这将所有结果分组为'3 - 4个月',这是不对的..