mysql下分组取关联表指定提示方法，类似于mssql中的cross apply

转至:https://stackoverflow.com/questions/12113699/get-top-n-records-for-each-group-of-grouped-results

通过分组的排序及序号获取条数信息，可以使用到索引，没测试性能，不知道和mssql的cross apply性能差异性为多少，只是能实现相应的效果。

#MySQL 5.7.12
#please drop objects you've created at the end of the script 
#or check for their existance before creating
#'\' is a delimiter


CREATE TABLE test
    (`Person` varchar(5), `Group` int, `Age` int)
;
    
INSERT INTO test
    (`Person`, `Group`, `Age`)
VALUES
    ('Bob', 1, 32),
    ('Jill', 1, 34),
    ('Shawn', 1, 42),
    ('Jake', 2, 29),
    ('Paul', 2, 36),
    ('Laura', 2, 39)
;

select person, `group`, age
from 
(
   select person, `group`, age,
      (@num:=if(@group = `group`, @num +1, if(@group := `group`, 1, 1))) row_number 
  from test t
  CROSS JOIN (select @num:=0, @group:=null) c
  order by `Group`, Age desc, person
) as x 
where x.row_number <= 2;


drop table test