如何限制 SQL 中每个字段值的行数?

2021-12-28 00:00:00 sql hive mysql greatest-n-per-group

例如，我在 Hive 中有一个这样的表:

For example, I have a table like this in Hive:

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

我只想返回第一列的每个唯一值的前两行.我希望这能够限制我从 Hive 传输到 MySQL 以用于报告目的的数据量.我想要一个给我这个的 HiveQL 查询:

and I want to only return the first two rows of each unique value of the first column. I want this to be able to limit the amount of data that I transfer from Hive into MySQL for reporting purposes. I'd like a single HiveQL query that gives me this:

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

推荐答案

不幸的是 mysql 没有分析函数.所以你必须玩弄变量.假设您有一个自动增量字段:

Unluckily mysql doesn't have Analytical Functions. So you have to play with variables. Supposing you have an autoincrement field:

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)

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