有限制的GROUP_CONCAT [英] GROUP_CONCAT with limit

问题描述

我有一个player -s与skill -s

I have table with player-s in many-to-many relation with skill-s

目标是通过单个查询列出球员及其前三项技能".

The goal is to list the players and their "top 3 skills" with a single query.

小提琴

create table player(
  id int primary key
);

create table skill(
  id int primary key,
  title varchar(100)
);

create table player_skills (
  id int primary key,
  player_id int,
  skill_id int,
  value int
);

查询:

SELECT 
p.id,  
group_concat(s.title  SEPARATOR ', ') as skills

FROM player p
LEFT JOIN player_skills ps ON ps.player_id = p.id
LEFT JOIN skill s ON s.id = ps.skill_id

WHERE ps.value > 2
-- skills limit 3 some how ...
group by p.id 
order by s.id


-- expected result
-- player_ID, skills
-- 1 , 'one'
-- 2 , 'one'
-- 3 , 'two, three, four'

正如您在小提琴中看到的那样，查询结果仅缺少3个技能的限制.
我尝试了几种子查询变体..联接等，但是没有效果.

As you can see in the fiddle the result of the query is missing only the limit of 3 skills.
I tried several variation of sub queries.. joins and so but with no effect.

推荐答案

一种比较古怪的方法是对GROUP_CONCAT的结果进行后处理:

One somewhat hacky way to do it is to post-process the result of GROUP_CONCAT:

substring_index(group_concat(s.title SEPARATOR ','), ',', 3) as skills

当然，这是假设您的技能名称不包含逗号，并且数量相当少.

Of course this assumes that your skill names don't contain commas and that their amount is reasonably small.

小提琴

针对GROUP_CONCAT的功能请求，以支持显式LIMIT不幸的是，该条款仍未解决.

A feature request for GROUP_CONCAT to support an explicit LIMIT clause is unfortunately still not resolved.

更新:由于用户草莓指出，表player_skills应该具有元组(player_id, skill_id)作为其主键，否则该架构允许将同一技能多次分配给玩家，在这种情况下group_concat将无法按预期工作.

UPDATE: As user Strawberry points out, the table player_skills should have the tuple (player_id, skill_id) as its primary key, otherwise the schema allows for the same skill to be assigned to a player multiple times, in which case group_concat would not work as expected.

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