Hoe dubbele rijen te verwijderen en de tabel bij te werken?

Om de besteltabel bij te werken, zoiets als:

update  orders
join    regions r1
on      r1.id = orders.region_id
set     orders.region_id = 
        (
        select  min(r2.id)
        from    regions r2
        where   r2.name = r1.name
        )

Daarna kunt u dubbele rijen verwijderen met:

delete  regions
from    regions
where   id not in
        (
        select  id
        from    (
                select  min(id) as id
                from    regions
                group by
                    name
                ) as SubqueryAlias
        )

De dubbele subquery is vereist om de MySQL-fout ERROR 1093 (HY000) at line 36: You can't specify target table 'regions' for update in FROM clause .