MariaDB 10.4

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create table tasks (id int, parent_id varchar(255), title varchar(255))
select 4 id, "#" parent_id, "LEVEL 2" title union all
select 5   , "4", "discovered check (level 2)" union all
select 6   , "4", "windmill (level 2)"              union all
select 7   , "#", "LEVEL 3"              union all
select 8   , "7", "discovered check - level 3"              union all
select 9   , "7", "windmill - level 3"              union all
select 10   , "#", "LEVEL 1"           union all
select 11   , "10", "discovered check - winning a piece (level 1)"              union all
select 12   , "10", "discovered check - mate in one (level 1)"              ;
select * from tasks;


id
parent_id
title


4
#
LEVEL 2

5
4
discovered check (level 2)

6
4
windmill (level 2)

7
#
LEVEL 3

8
7
discovered check - level 3

9
7
windmill - level 3

10
#
LEVEL 1

11
10
discovered check - winning a piece (level 1)

12
10
discovered check - mate in one (level 1)


create table puzzles (id int, task_id int)
select  52 id, 5 task_id union all
select  61 id, 6 task_id union all
select  25 id, 6 task_id union all
select  70 id, 11 task_id union all
select  53 id, 12 task_id union all
select  62 id, 9 task_id union all
select  27 id, 9 task_id union all
select  72 id, 8 task_id;
select * from puzzles;


id
task_id


52
5

61
6

25
6

70
11

53
12

62
9

27
9

72
8


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 


task_id
id


11
70

5
52

8
72


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 


task_id
id


12
53

5
52

8
72


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 


task_id
id


12
53

5
52

8
72

id	parent_id	title
4	#	LEVEL 2
5	4	discovered check (level 2)
6	4	windmill (level 2)
7	#	LEVEL 3
8	7	discovered check - level 3
9	7	windmill - level 3
10	#	LEVEL 1
11	10	discovered check - winning a piece (level 1)
12	10	discovered check - mate in one (level 1)