执行联接时，将表列值用作LIMIT [英] Use table column value for LIMIT when performing join

问题描述

在一种情况下，我正在两个表之间执行联接，并且我需要一个表中的值用作联接中子查询的LIMIT因子.假设我有以下[极其简化的]表-

I have a situation where I'm performing a join between two tables, and I need a value from one table to be used as a LIMIT factor for a subquery in the join. Assume I have the following [extremely simplified] tables -

data:

experiment_id | value
--------------|--------
       1      |  2.5
       1      |  2.6
       1      |  4.5
       1      |  2.3
       1      |  3.5
       1      |  2.8
       2      |  2.3
       2      |  1.2
       2      |  1.1
       2      |  3.6
       2      |  3.8
       2      |  4.1
       2      |  7.9
       2      |  4.2
       2      |  1.0


data_clip:

experiment_id | clip_index
--------------|------------
       1      |     3
       2      |     5

我需要将每个实验的已排序值相加到一个特定的clip_index，该值在实验之间会有所不同.因此，理想情况下，我的结果表将如下所示:

I need to sum each experiment's sorted values up to a certain clip_index, that varies between experiments. So, my result table will ideally look like this:

results:
experiment_id |  sum
--------------|-------
       1      |  7.6    # => 2.3 + 2.5 + 2.8
       2      | 13.0    # => 1.0 + 1.1 + 1.2 + 2.3 + 3.6 + 3.8 

通常，我会使用一些客户端脚本(红宝石，python等)进行此计算，但是我想尝试在数据库级别进行此计算.一些虚构的SQL可能看起来像这样(我知道此查询存在种种错误，但希望您能理解这个主意):

Typically, I'd do this calculation with some client side scripting (ruby, python, etc), but I wanted to try doing this on the db level. Some imaginary SQL might look like this (there's all sorts of wrong with this query, I know, but hopefully you get the idea):

SELECT 
  T0.experiment_id as `id`,
  (SELECT SUM(x.value) from
       (SELECT   value 
        FROM     data 
        WHERE    experiment_id = t0.experiment_id
        ORDER BY value 
        LIMIT    t0.clip_index ) as x) AS `sum`
FROM data_clip AS t0

几个问题:

1. 必须使用常量(1000、10等)而不是列来定义LIMIT.
2. 子查询中的WHERE条件由于无法识别子查询外部的t0表而失败.

1. LIMIT must be defined with a constant (1000, 10, etc.), not a column.
2. the WHERE condition in the subquery fails for not recognizing t0 table, which is external to the sub-query.

我的问题基本上是如何使用SQL来完成两个表之间的变量限制和求和.我曾考虑过使用group_concat和substring_index来将每一行的值隔离为clip_index，但是这是一个问题，即对带数字的字符串("1.2,2.3,3.2")进行求和以及服务器对缓冲区(可配置，但每个实验的值大约为100k).有什么想法吗?谢谢.

My question is basically how to accomplish the variable limit and sum between the two tables using mostly SQL. I thought about using group_concat and substring_index to isolate the values up to clip_index for each row, but then there's the issue of summing up the numbered strings ("1.2,2.3,3.2") and the server limitation on the size of group_concat buffer (configurable, but values can be around ~100k per experiment). Any thoughts? Thanks.

推荐答案

我想您只需要在选择的每个值中包含一个行号，然后将结果限制为行数 像这样的东西: (未经测试)

I guess you just need to include a row number with each value selected and limit the results by the number of rows something like this: (not not tested)

SELECT T0.experiment_id as `id`,   
(SELECT SUM(x.value) from       
 (SELECT value,@rownum := @rownum + 1 AS rownum           
  FROM data         
  JOIN (SELECT @rownum := 0) r
  WHERE experiment_id = t0.experiment_id         
  ORDER BY value             
 ) AS x
WHERE x,rownum < t0.clip_index
) AS `sum`
 FROM data_clip AS t0

请参阅: MySQL-在选择时获取行号

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