按id降序查询几种情况的各一条记录，用一条语句能否实现？
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union 可不可以的，没用过
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对
(select * from a order by id desc limit 0,1) union all (select * from b order by id desc order by id desc limit 0,1)
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可以啊，用UNION ALL
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按情况分组 每组取一条记录
- SQL code
SELECT * FROM (SELECT * FROM `table` ORDER BY `condition` DESC) t GROUP BY `condition`;