使用时GROUP BY，结果的每一行对应于原始表的多行。这些结果的计算有三种可能性：
要解决您的问题，您必须首先使用aGROUP BY获取足够的信息以标识所需的记录：
SELECT smsCONID, -- OK: used in GROUP BY
MAX(smsTIMESTAMP) AS smsTIMESTAMP, -- OK: aggregate MAX
COUNT(*) AS smsNUMMESSAGES -- OK: aggregate COUNT
FROM sms
GROUP BY smsCONID
ORDER BY smsTIMESTAMP DESC
然后，将该结果表与原始sms表连接起来以获取这些记录的其他列：
SELECT *
FROM (SELECT smsCONID,
MAX(smsTIMESTAMP) AS smsTIMESTAMP,
COUNT(*) AS smsNUMMESSAGES
FROM sms
GROUP BY smsCONID) AS grouped
JOIN sms
ON grouped.smsCONID = sms.smsCONID
AND grouped.smsTIMESTAMP = sms.smsTIMESTAMP
ORDER BY smsTIMESTAMP DESC
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