group by后取最近记录
表testtb 中news_id对应有多条数据，只取news_id一条最新数据，即creatdate最大的
1 2 | SELECT t1.id, t1.news_id, t1.username, t1.creatdate FROM (SELECT id,news_id,username,creatdate FROM testtb ORDER BY creatdate DESC) t1 GROUP BY t1.creatdate |
最后说，下面这样是得不到正确结果的
1 | SELECT id,news_id,username,creatdate FROM testtb GROUP BY creatdate ORDER BY creatdate DESC |
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