如何限制SQL中每个字段值的行数？

Question

例如，我有一个这样的表蜂巢：

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

，我想只返回第一列的每个唯一值的前两行。我希望这可以限制从Hive传输到MySQL中用于报告目的的数据量。我想单个HiveQL查询给我这个：

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

Answer 1

不幸的是mysql没有解析函数。所以你必须玩变数。 假设你有一个自动增量字段：

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

蜂房的解决办法是

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