mysql - SQL 删除除每个唯一值的最后 N 行之外的所有行

Question

这是一个艰难的， 对于不同字段中的每个唯一值，我将如何删除除最后一行(比如 3 行)以外的所有内容？

这是问题的视觉图:

id | otherfield
---------------
 1 | apple      <- DELETE
 2 | banana     <- KEEP
 3 | apple      <- DELETE
 4 | apple      <- KEEP
 5 | carrot     <- KEEP
 6 | apple      <- KEEP
 7 | apple      <- KEEP
 8 | banana     <- KEEP

我如何在 SQL 中完成此操作？

Answer 1

未经测试，但按照这些思路可能会起作用:

DELETE t.*
FROM table t JOIN (
    SELECT id
        @rowNum := IF(@otherfield <> otherfield, 1, @rowNum + 1) rn,
        @otherfield := otherfield otherfield
    FROM (
        SELECT id, otherfield
        FROM table
        ORDER BY otherfield, id DESC
    ) t, (SELECT @otherfield := NULL, @rowNum := -1) dm
) rs ON t.id = rs.id
WHERE rs.rn > 3