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