Edit size 2 (Fail: Subquery)

SQL

use zhhanswiki_p;

set @mobile_edit_tag = (
  select
    ctd_id
  from
    change_tag_def
  where
    ctd_name = 'mobile edit'
);

select
  substring(rev_timestamp, 1, 4) as year,
  sum(ct_rev_id is not null) as mobile_revisions,
  count(rev_id) as revisions,
  concat(
    if(
      count(rev_id) = 0,
      0,
      format(
        sum(ct_rev_id is not null) / count(rev_id) * 100.0,
        2
      )
    ),
    '%'
  ) as mobile_percent
from
  revision
left join
  change_tag
  on
    ct_rev_id = rev_id and
    ct_tag_id = @mobile_edit_tag
where
  rev_timestamp like '2020%'
group by
  year
order by
  year asc;
SQL

Checking query status...
