mysql - MySQL：从表中删除除 20 行之外的所有行？

Question

我有一张有数十万行的表，分配给一个用户。例如：

itemid | userid | etc
1      | 1      | etc
2      | 1      | etc
3      | 1      | etc
4      | 3      | etc
5      | 3      | etc
6      | 3      | etc
etc    | etc    | etc

用户可以有任意数量的项目分配给他或她。即从0 到无穷大的任何数字。我的问题是我想要一个 SQL 查询，它将删除每个用户的所有项目，但保留 20 个。如果用户少于 20 个，例如，只分配了 10 个项目，它必须保留所有 10 个。

我怎么做？

更新

如果用户有 50 个项目，ID 为 1 - 50，则它必须返回项目 30 - 50。换句话说，该用户的最后 20 个插入项目。

Answer 1

假设我们要删除每个用户的两个最新条目之外的所有条目......

CREATE TABLE my_table(itemid INT NOT NULL AUTO_INCREMENT PRIMARY KEY,userid INT NOT NULL);

INSERT INTO my_table VALUES
(1, 1),
(2, 1),
(3, 1),
(4, 3),
(5, 3),
(6, 3);

SELECT * FROM my_table;
+--------+--------+
| itemid | userid |
+--------+--------+
|      1 |      1 |
|      2 |      1 |
|      3 |      1 |
|      4 |      3 |
|      5 |      3 |
|      6 |      3 |
+--------+--------+

这是一个选择要删除的行的查询...

SELECT a.* 
  FROM my_table a 
  LEFT 
  JOIN 
     ( SELECT x.*, COUNT(*) FROM my_table x JOIN my_table y ON y.userid = x.userid AND y.itemid >= x.itemid GROUP BY x.itemid HAVING COUNT(*) <=2)b 
    ON b.itemid = a.itemid 
 WHERE b.itemid IS NULL;
+--------+--------+
| itemid | userid |
+--------+--------+
|      1 |      1 |
|      4 |      3 |
+--------+--------+

...这是删除它们的查询...

DELETE a 
  FROM my_table a 
  LEFT 
  JOIN 
     ( SELECT x.*, COUNT(*) FROM my_table x JOIN my_table y ON y.userid = x.userid AND y.itemid >= x.itemid GROUP BY x.itemid HAVING COUNT(*) <=2)b 
    ON b.itemid = a.itemid 
 WHERE b.itemid IS NULL;

SELECT * FROM my_table;
+--------+--------+
| itemid | userid |
+--------+--------+
|      2 |      1 |
|      3 |      1 |
|      5 |      3 |
|      6 |      3 |
+--------+--------+

Answer 2

尝试这个

     delete from Fruits where itemid  
     not in (select itemid from (select itemid from Fruits where userid = 1
     ORDER BY itemid desc limit 20)x  )
     and userid=1

在这里演示

Answer 3

该代码的工作原理如下：

• 创建一个新列：count - 计算每个用户的项目
• 按 itemid 降序排列结果 - 以便用户的第一个事件获得最高 id
• 选择/删除所有计数 > 20 的行 - 保持最后 20 个（或更少）事件不变

选择：

SET @count=1;
SET @last_user_id=0;

SELECT * FROM (
SELECT *,
case 
when (@last_user_id = userid) 
then 
@count:=@count+1
else @count:=1 end as count,
case 
when (@last_user_id!=userid) 
then 
@last_user_id:=userid
end
as new
FROM <tablename> ORDER BY userid, itemid DESC) AS inner_table WHERE count > 20

删除：

SET @count=1;
SET @last_user_id=0;

DELETE FROM (
SELECT *,
CASE 
WHEN (@last_user_id = userid) 
THEN  
@count:=@count+1
ELSE 
@count:=1 
END 
AS count,
CASE 
WHEN (@last_user_id!=userid) 
THEN  
@last_user_id:=userid
END 
AS new
FROM <tablename> ORDER BY userid, itemid DESC) AS inner_table WHERE count > 20

Answer 4

这对我有用：

DELETE FROM table WHERE itemid NOT IN ( 
      SELECT itemid 
        FROM ( 
          SELECT * 
          FROM table
          WHERE userid = 1
          ORDER BY itemid  DESC 
          LIMIT 20
        ) x 
    ) 
    AND userid = 1