MySQL update join with limit

Question

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 )

My table structure looks like this

wp_postmeta

meta_id, post_id, meta_key, meta_value

wp_map

oldmap, newmap

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.

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;

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.

Answer 1

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;

Here is SQLFiddle demo