How to delete duplicate rows with SQL?

DELETE FROM table WHERE id NOT IN
    (SELECT MAX(id) FROM table GROUP BY date);

I don't have comment rights, so here's my comment as an answer in case anyone comes across the same problem:

In SQLite3, there is an implicit numerical primary key called "rowid", so the same query would look like this:

DELETE FROM table WHERE rowid NOT IN
(SELECT MAX(rowid) FROM table GROUP BY date);

this will work with any table even if it does not contain a primary key column called "id".

For mysql,postgresql,oracle better way is SELF JOIN.

Postgresql:
DELETE FROM table t1 USING table t2 WHERE t1.date=t2.date AND t1.id<t2.id;

MySQL        
DELETE FROM table
USING table, table as vtable
WHERE (table.id < vtable.id)
AND (table.date=vtable.date)

SQL aggregate (max,group by) functions almost always are very slow.