MySQL更新加入限制 [英] MySQL update join with limit

问题描述

我想知道如何使用另一个表中的值来更新一个表，诀窍是我需要设置一个限制，因为我有成千上万的行要更新，而PHPmyadmin无法处理该负载. (我没有直接访问服务器的权限)

I would like to know how to update a table, with values from another table, by the trick is I need to set a limit because I have thousands of rows to update, and PHPmyadmin can't handle that load. ( I dont have direct access to the server )

我的表结构如下

wp_postmeta

meta_id, post_id, meta_key, meta_value

meta_id, post_id, meta_key, meta_value

wp_map

oldmap, newmap

我需要做的是联接wp_postmeta.meta_value和wp_map.oldmap上的两个表，并用wp_map.newmap更新wp_postmeta.meta_value.

What I need to do is join the two tables on wp_postmeta.meta_value and wp_map.oldmap, and update the wp_postmeta.meta_value with wp_map.newmap.

这是我当前的查询，但是我需要在查询中添加100的LIMIT，因为我将查询分为较小的块，以便PHPMyAdmin可以处理.

Here is my current query, but I need to add a LIMIT of 100 to the query, as I'm splitting the query up into smaller chunks so PHPMyAdmin can process.

UPDATE wp_postmeta
INNER JOIN wp_map
ON wp_map.oldmap = wp_postmeta.meta_value
SET wp_postmeta.meta_value = wp_map.newmap;

我读过有关创建子查询的信息，但找不到任何相关示例，因此，如果有人可以引导我朝正确的方向或提供有效的示例，将不胜感激.

I read about creating a subquery, but couldn't find any relevant examples, so if someone could steer me in the right direction or provide a working example it would be greatly appreciated.

推荐答案

您可以通过这种方式尝试

You can try it this way

UPDATE wp_postmeta t JOIN
(
    SELECT p.meta_id, m.newmap
      FROM wp_postmeta p JOIN wp_map m
        ON p.meta_value = m.oldmap
     ORDER BY p.meta_id
     LIMIT 100
) s
   ON t.meta_id = s.meta_id
  SET t.meta_value = s.newmap;

这里是 SQLFiddle 演示

Here is SQLFiddle demo

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