mysql - SQL query: Delete all records from the table except latest N?

Question

Is it possible to build a single mysql query (without variables) to remove all records from the table, except latest N (sorted by id desc)?

Something like this, only it doesn't work :)

delete from table order by id ASC limit ((select count(*) from table ) - N)

Thanks.

Answer 1

You cannot delete the records that way, the main issue being that you cannot use a subquery to specify the value of a LIMIT clause.

This works (tested in MySQL 5.0.67):

DELETE FROM `table`
WHERE id NOT IN (
  SELECT id
  FROM (
    SELECT id
    FROM `table`
    ORDER BY id DESC
    LIMIT 42 -- keep this many records
  ) foo
);

The intermediate subquery is required. Without it we'd run into two errors:

1. SQL Error (1093): You can't specify target table 'table' for update in FROM clause - MySQL doesn't allow you to refer to the table you are deleting from within a direct subquery.
2. SQL Error (1235): This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery' - You can't use the LIMIT clause within a direct subquery of a NOT IN operator.

Fortunately, using an intermediate subquery allows us to bypass both of these limitations.

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Nicole has pointed out this query can be optimised significantly for certain use cases (such as this one). I recommend reading that answer as well to see if it fits yours.