如果您没有将整个CASE语句GROUP BY包装在子查询中，则需要使用该语句中的整个语句。
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
ON person.EntityID = o.id
INNER JOIN Organization AS org
ON o.OrganizationID = Org.ID
WHERE person.TitleID = 2
AND o.bID = 1
GROUP BY 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
ORDER BY Length ASC;
还要删除ORDER BY子句中列名周围的单引号。
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