要求：获得按 table1_id 分组，并且取得 table2 中 age 最大的记录信息。

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)