如何使用 DELETE FROM 和子查询删除重复项？

Question

Notes:

• The return from SELECT version() is 10.5.12-MariaDB-log
• Default collation: utf8mb4_unicode-ci
• Default charset: utf8mb4
• Queries run using MySQL Workbench for Ubuntu Linux 8.0.29

My goal is to delete duplicated items in a table. I do not have any other table to use as a reference to check duplicates. I created a simple query and subquery that returns expected results:

    SELECT * FROM messages WHERE id NOT IN
        (SELECT id FROM
            messages
        WHERE
            uid = '11899414026778263'
        GROUP BY message_id , uid
        ORDER BY created_at);

Despite setting SQL_SAFE_UPDATES to 0, a DELETE operation using the same data fails. I get a "QUERY Interrupted" message.

    SET SQL_SAFE_UPDATES = 0;
    DELETE FROM messages WHERE id NOT IN
        (SELECT id FROM
            messages
        WHERE
            uid = '11899414026778263'
        GROUP BY message_id , uid
        ORDER BY created_at);
    SET SQL_SAFE_UPDATES = 1;

If I replace DELETE with SELECT * , the query returns results. Another StackOverflow answer said that querying based on a sub-query does not work in MySQL. Others say to use another table as reference instead of a subquery. DELETE query results in 'Query Interrupted' MySQL Workbench?

This method works in some SQL implementations based on these answers and websites:

• Delete Duplicates From a Table in SQL Server
• Different ways to SQL delete duplicate rows from a SQL Table
• How to delete duplicate rows in SQL Server?

Answer 1

order by is applied after grouping, so your order by is not sufficient to select the id with the lowest created_at for each group. Your query will fail under ONLY_FULL_GROUP_BY because the id returned by the subselect will be arbitrary within each group. You want to use first_value instead.

But it's easier just to not use a subquery:

delete m
from messages m
# is there a message we would prefer over this one?
inner join messages m2 on (m2.uid,m2.message_id)=(m.uid,m.message_id) and (m2.created_at,m2.id) < (m.created_at,m.id)