要求：获得按table1_id分组，并且age最大的记录信息，即2、3、5条
方法一：
select * from (select * from table2 order by age desc) as a
group by a.table1_id
方法二：
select a.* from table2 as a where age = (select max(age) from
table2 where a.table1_id=table1_id)
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)
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)