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要求：获得按table1_id分组，并且age最大的记录信息，即2、3、5条
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方法一：
select * from (select * from table2?order by age desc) as a group by a.table1_id
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方法二：
select a.* from table2 as a where age = (select max(age) from table2 where a.table1_id=table1_id)
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方法三：
select a.* from table2 as a where not exists (select * from table2 where table1_id=a.table1_id and age>a.age)
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方法四：
select a.* from table2 as a where exists (select count(*) from table2 where table1_id=a.table1_id and age>a.age having count(*)=0)
来源：?http://blog.sina.com.cn/s/blog_8155e74d0101g1pl.html