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create table posts ( id int, post varchar(64) ); insert into posts values (1, 'P1'), (2, 'P2'); create table tags ( id int, tag varchar(64) ); insert into tags values (1, 'T1'), (2, 'T1'), (3, 'T2'); create table post_tag ( post_id int, tag_id int ); insert into post_tag values (1, 1), (2, 2), (2, 3); select * from post_tag; update post_tag join tags on tags.id = tag_id join ( select tag, min(id) id from tags group by tag ) gtag on gtag.tag = tags.tag set post_tag.tag_id = gtag.id; select * from post_tag; delete tags from tags join tags t2 on t2.tag = tags.tag and tags.id > t2.id ; select * from tags; create unique index tag_uq on tags(tag);