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.