如何从子表中选择一个随机ID？

如何解决如何从子表中选择一个随机ID？

我正在使用10.4.13-MariaDB。我有一个父表和一个子表。

父表（任务）

id parent_id
4    # <- Main (parent theme) Level 2
5    4 <- child `discovered check` (level 2)
6    4 <- child  `windmill`  (level 2)
7    # <- Main (parent theme) Level 3
8    7 <- child `discovered check - level 3` (level 3)
9    7 <- child  `windmill - level 3`  (level 3)
10    # <- Main (parent theme) Level 1
11    10 <- child `discovered check - winning a piece` (level 1)
12    10 <- child  `discovered check - mate in one`  (level 1)

界面看起来像

我有一个子表（拼图）-拼图表-拼图只能属于子主题

 id      task_id
 52         5 <- belongs to `discovered check` (level 2)
 61         6 <- belongs to `windmill` (level 2)
 25         6 <- belongs to `windmill` (level 2)
 70         11 <- belongs to `discovered check - winning a piece` (level 1)
 53         12 <- belongs to `discovered check - mate in 1` (level 1)
 62         9 <- belongs to `windmill - level 3` (level 3)
 27         9 <- belongs to `windmill - level 3` - (level 3)
 72         8 <- belongs to `discovered check - level 3` - (level 3)

我想从每个级别中获得一个随机拼图。如何编写适当的查询？

预期结果：

task_id  id
  5      52 <- random puzzle from level 2
  11     70 <- random puzzle from level 1
  8      72 <- random puzzle from level 3

我的小提琴：https://dbfiddle.uk/?rdbms=mariadb_10.4&fiddle=7bed2a19a0f98abccbe06ba9e0ae358b

解决方法

WITH cte AS ( SELECT tasks.id task_id,puzzles.puzzle_id,row_number() over (partition by tasks.id order by rand()) rn
              FROM tasks
              JOIN puzzles ON tasks.id = puzzles.task_id )
SELECT task_id,puzzle_id
FROM cte
WHERE rn = 1;

需要8.0+版本或MariaDB 10.2 +

select tasks.id task_id,rand_puzzle.puzzle_id
from tasks,lateral ( select puzzle_id
               from puzzles
               where tasks.id = puzzles.task_id 
               order by rand() limit 1 ) rand_puzzle;

需要版本8.0.14 +。

fiddle

如果您需要某些子树的数据，请仅从以前的递归CTE中的tasks表中提取该子树。

每个主要任务需要一个难题。因此，首先（使用递归查询）从层次上查找属于主任务的所有任务，然后加入谜题并为每个主任务选择一个。

（主要任务是您在请求中称为“级别”。）

with recursive cte (main_task_id,task_id) as
(
  select parent_id,task_id
  from tasks
  where parent_id in (select id from tasks where parent_id is null)
  union all
  select cte.main_task_id,t.task_id
  from cte
  join tasks t on t.parent_id = cte.task_id
),randomly_numbered as
(
  select
    cte.main_task_id,p.task_id,p.id as puzzle_id,row_number() over (partition by main_task_id order by rand()) as rn
  from cte
  join puzzle p on p.task_id = cte.task_id
)
select
  -- main_task_id,task_id,puzzle_id
from randomly_numbered
where rn = 1
order by main_task_id;

具有ROW_NUMBER()窗口功能：

with cte as (
  select p.*,row_number() over (partition by t.parent_id order by rand()) rn
  from puzzles p inner join tasks t  
  on t.id = p.task_id
)
select task_id,id
from cte 
where rn = 1

请参见demo。

row_number()的分区应按parent_id的列tasks进行划分，因此每个主题的结果都有1行。