¿Cómo puedo limitar el número de filas por valor de campo en SQL?

Question

Por ejemplo, tengo una tabla como esta en la colmena:

1 1 
1 4 
1 8 
2 1 
2 5 
3 1 
3 2 

y quiero sólo para volver las dos primeras filas de cada valor único de la primera columna. Quiero que esto sea capaz de limitar la cantidad de datos que transfiero de Hive a MySQL para fines de informes. Quisiera una sola consulta HiveQL que me diga esto:

1 1 
1 4 
2 1 
2 5 
3 1 
3 2 

Answer 1

Desgraciadamente mysql no tiene funciones analíticas. Entonces debes jugar con variables. suponiendo que tiene un campo de incremento automático:

mysql> create table mytab (
    -> id int not null auto_increment primary key, 
    -> first_column int, 
    -> second_column int 
    ->) engine = myisam; 
Query OK, 0 rows affected (0.05 sec) 

mysql> insert into mytab (first_column,second_column) 
    -> values 
    -> (1,1),(1,4),(2,10),(3,4),(1,4),(2,5),(1,6); 
Query OK, 7 rows affected (0.00 sec) 
Records: 7 Duplicates: 0 Warnings: 0 

mysql> select * from mytab order by id; 
+----+--------------+---------------+ 
| id | first_column | second_column | 
+----+--------------+---------------+ 
| 1 |   1 |    1 | 
| 2 |   1 |    4 | 
| 3 |   2 |   10 | 
| 4 |   3 |    4 | 
| 5 |   1 |    4 | 
| 6 |   2 |    5 | 
| 7 |   1 |    6 | 
+----+--------------+---------------+ 
7 rows in set (0.00 sec) 

mysql> select 
    -> id, 
    -> first_column, 
    -> second_column, 
    -> row_num 
    -> from (
    -> select *, 
    -> @num := if(@first_column = first_column, @num:= @num + 1, 1) as row_num, 
    -> @first_column:=first_column as c 
    -> from mytab order by first_column,id) as t,(select @first_column:='',@num: 
=0) as r; 
+----+--------------+---------------+---------+ 
| id | first_column | second_column | row_num | 
+----+--------------+---------------+---------+ 
| 1 |   1 |    1 |  1 | 
| 2 |   1 |    4 |  2 | 
| 5 |   1 |    4 |  3 | 
| 7 |   1 |    6 |  4 | 
| 3 |   2 |   10 |  1 | 
| 6 |   2 |    5 |  2 | 
| 4 |   3 |    4 |  1 | 
+----+--------------+---------------+---------+ 
7 rows in set (0.00 sec) 

mysql> select 
    -> id, 
    -> first_column, 
    -> second_column, 
    -> row_num 
    -> from (
    -> select *, 
    -> @num := if(@first_column = first_column, @num:= @num + 1, 1) as row_num, 
    -> @first_column:=first_column as c 
    -> from mytab order by first_column,id) as t,(select @first_column:='',@num: 
=0) as r 
    -> having row_num<=2; 
+----+--------------+---------------+---------+ 
| id | first_column | second_column | row_num | 
+----+--------------+---------------+---------+ 
| 1 |   1 |    1 |  1 | 
| 2 |   1 |    4 |  2 | 
| 3 |   2 |   10 |  1 | 
| 6 |   2 |    5 |  2 | 
| 4 |   3 |    4 |  1 | 
+----+--------------+---------------+---------+ 
5 rows in set (0.02 sec) 

Answer 2

solución de una colmena sería

SELECT S.col1, S.col2 
FROM 
(SELECT col1, col2, row_number() over (partition by col1) as r FROM mytable) S 
WHERE S.r < 3