MariaDB 10.3

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create table mytable (client int, data json);
insert into mytable values (1, '["a","b","c"]'), (2, '["k"]')

Records: 2  Duplicates: 0  Warnings: 0


create table bigtable as
select 100 x union all select 200 union all select 300 union all select 400 union all select 500;

Records: 5  Duplicates: 0  Warnings: 0


select client, json_unquote(json_extract(t.data, concat('$[', n.rn - 1, ']'))) value
from mytable t
inner join (select row_number() over() rn from bigtable) n
    on n.rn <= json_length(t.data)
order by t.client, n.rn


client
value


1
a

1
b

1
c

2
k