如何在mysql中编写这个特定的查询？

Question

我有一段 SQL 代码取自 SQL Server 下的 ELMAH 存储过程：

DELETE FROM [ELMAH_Error]
WHERE ErrorId NOT IN (SELECT TOP 100 ErrorId FROM ELMAH_Error ORDER BY TimeUtc DESC)

我现在迁移一个网站改用mysql，不知道怎么翻译这段代码。

我必须在这段代码之后添加翻译：

CREATE DEFINER=`b43638b86659eb`@`%` PROCEDURE `elmah_LogError`(
    IN ErrorId CHAR(36), 
    IN Application varchar(60), 
    IN Host VARCHAR(30), 
    IN Type VARCHAR(100), 
    IN Source VARCHAR(60), 
    IN Message VARCHAR(500), 
    IN User VARCHAR(50), 
    IN AllXml TEXT, 
    IN StatusCode INT(10), 
    IN TimeUtc DATETIME
)
    MODIFIES SQL DATA
BEGIN
    INSERT INTO `elmah_error` (
        `ErrorId`, 
        `Application`, 
        `Host`, 
        `Type`, 
        `Source`, 
        `Message`, 
        `User`, 
        `AllXml`, 
        `StatusCode`, 
        `TimeUtc`
    ) VALUES (
        ErrorId, 
        Application, 
        Host, 
        Type, 
        Source, 
        Message, 
        User, 
        AllXml, 
        StatusCode, 
        TimeUtc
    );
END

我尝试在 END 语句之前添加@Sick 的响应，但 SQL Workbench（通过 alter procedure 命令）看不到它。

谢谢

Answer 1

Sql Server TOP 的等效项在 Mysql 中是 Limit 并且应该在查询的末尾

DELETE FROM `ELMAH_Error`
WHERE ErrorId NOT IN (SELECT ErrorId 
                      FROM `ELMAH_Error`
                      ORDER BY TimeUtc DESC LIMIT 100)

Answer 2

如果表真的很大，那么在 MySQL 中截断通常比删除快得多。比如：

lock tables ELMAH_Error write;
create temporary table cpy_ELMAH_Error 
   select * from ELMAH_Error order by TimeUtc DESC LIMIT 100;
truncate table ELMAH_Error;
insert into  ELMAH_Error select * from cpy_ELMAH_Error;
drop table cpy_ELMAH_Error;
unlock tables;

Answer 3

如果您有兴趣，还有一种更简单的方法：

set @N=0;
delete from ELMAH_Error where (@N:=@N+1) > 101 order by TimeUtc DESC;

Answer 4

MySQL 在某些子查询中不支持limit。如果是这种情况，则使用显式连接：

DELETE e
    FROM ELMAH_Error e LEFT JOIN
         (SELECT ErrorId
          FROM ELMAH_Error
          ORDER BY TimeUtc DESC
          LIMIT 100
         ) ee
         ON ee.ErrorId = e.ErrorId
    WHERE ee.ErrorId IS NULL;

编辑：

上面的代码假定ErrorId 在表中是唯一的并且没有NULL 值。如果这不是真的，那么查询将不起作用。 （注意：假设名为 Id 的列是唯一的且不为 null 的假设似乎非常合理。）

DELETE e
    FROM ELMAH_Error e LEFT JOIN
         (SELECT TimeUtc
          FROM ELMAH_Error
          ORDER BY TimeUtc DESC
          LIMIT 100
         ) ee
         ON ee.TimeUtc = e.TimeUtc
    WHERE ee.TimeUtc IS NULL;