重複したタグを削除するMySQL

セットアップ

create table overly_complex_tags
(
  id integer primary key not null,
  tags varchar(100) not null
);

insert into overly_complex_tags
( id, tags )
values
( 3   , 'x,yz,z,x,x'           ),
( 5   , 'a,b,c d,a,b,c d,d'    )
;

create view digits_v
as
SELECT 0 AS N 
UNION ALL 
SELECT 1 
UNION ALL 
SELECT 2 
UNION ALL 
SELECT 3 
UNION ALL 
SELECT 4 
UNION ALL 
SELECT 5 
UNION ALL 
SELECT 6 
UNION ALL 
SELECT 7 
UNION ALL 
SELECT 8 
UNION ALL 
SELECT 9
;

クエリ削除重複タグ

update overly_complex_tags t
inner join
(
select id, group_concat(tag) as new_tags
from
(
select distinct t.id, substring_index(substring_index(t.tags, ',', n.n), ',', -1) tag
from overly_complex_tags t 
cross join
(
  select a.N + b.N * 10 + 1 n
  from digits_v a
  cross join digits_v b
  order by n
) n
where n.n <= 1 + (length(t.tags) - length(replace(t.tags, ',', '')))
) cleaned_tags
group by id
) updated_tags
on t.id = updated_tags.id
set t.tags = updated_tags.new_tags
;

出力

+----+-----------+
| id |   tags    |
+----+-----------+
|  3 | yz,z,x    |
|  5 | c d,a,d,b |
+----+-----------+

sqlfiddle

注