1.开窗函数over partition
开窗函数使用于取出多列分组，取一列分组下另一组前几名，先利用开窗函数对其分组排名，开窗函数排名函数较多使用row_number()，还有rank()等，生成排名列之后将结果集筛选其排名前几<=2或者你想要的前几名。
上述例子sql：
select test_name,test_id,n_rank,test_salary_sum from ( select t_alias.test_name,t_alias.test_id, sum(t_alias.test_salary) test_salary_sum, row_number() over(partition by t_alias.test_name order by sum(t_alias.test_salary) desc) n_rank from (select 1 test_id, 'name_1' test_name, 100 test_salary from dual union all select 2 test_id, 'name_1' test_name, 200 test_salary from dual union all select 2 test_id, 'name_1' test_name, 300 test_salary from dual union all select 3 test_id, 'name_1' test_name, 300 test_salary from dual union all select 4 test_id, 'name_1' test_name, 400 test_salary from dual union all select 1 test_id, 'name_2' test_name, 200 test_salary from dual union all select 2 test_id, 'name_2' test_name, 200 test_salary from dual union all select 2 test_id, 'name_2' test_name, 200 test_salary from dual union all select 3 test_id, 'name_2' test_name, 400 test_salary from dual union all select 4 test_id, 'name_2' test_name, 400 test_salary from dual) t_alias group by t_alias.test_name,t_alias.test_id) where n_rank<=2 order by test_name,n_rank;
上述执行结果：
获得每个test_name字段下test_id的test_salary的前2名。
开窗函数实例：
另有开窗函数替代写法,可以了解：
