MySQL 5.7

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create  table numbers (
  n int );

insert into numbers values (1),(2),(3),(4);

create table tab (
  id int ,
  relatedIdInASet varchar(255));

insert into tab values
(1,'[12,34,56]'),
(2,'[11,12,22,34]');

Records: 4  Duplicates: 0  Warnings: 0

Records: 2  Duplicates: 0  Warnings: 0

select * from numbers;
select * from tab;

n

1

2

3

4

id
relatedIdInASet

1
[12,34,56]

2
[11,12,22,34]

update tab
set relatedIdInASet = replace(replace(relatedIdInASet, '[', ''),']','');

Rows matched: 2  Changed: 2  Warnings: 0

select * from tab;

id
relatedIdInASet

1
12,34,56

2
11,12,22,34

select 
  id, 
  substring_index( substring_index(relatedIdInASet, ',', n),',', -1) as relatedIdInASet
from tab
join numbers
  on char_length(relatedIdInASet) 
    - char_length(replace(relatedIdInASet, ',', '')) 
    >= n - 1;

id
relatedIdInASet

1
12

2
11

1
34

2
12

1
56

2
22

2
34

create table tab2 as select 
  id, 
  substring_index( substring_index(relatedIdInASet, ',', n),',', -1) as relatedIdInASet
from tab
join numbers
  on char_length(relatedIdInASet) 
    - char_length(replace(relatedIdInASet, ',', '')) 
    >= n - 1 ;

Records: 7  Duplicates: 0  Warnings: 0

select * from tab2;

id
relatedIdInASet

1
12

2
11

1
34

2
12

1
56

2
22

2
34