Question

I want to perform some updates to posts’ meta data in a WordPress site (wp_postmeta table).
The known good’ol function update_post_meta() is doing the job and also will check and if key not exist, it will add it.
But, this function can handle only one post at a time.
I want to save expensive db calls and update with one query several fields for different post ids.

Generally, I could use mySQL UPDATE query with CASE to swich and assign each meta_value according to post_id, like in this example, and update some values of different rows in the table.
But, it may be that for some of the posts, that meta_key is not existing yet, so INSERT is needed and not update. Well, if not using the update_post_meta(), I should check this somehow.
I tried to use INSERT .... ON DUPLICATE UPDATE (syntax) and also REPLACE (syntax), but it is relevant only for columns that are unique or defined as primary keys, so it doesn’t help in this case.

Is there a way or an idea for how can I have the “Insert value, and update if already exists this meta_key for this post_id” and do this for several different post_ids (same meta_key however all the way), and with one single mySQL query?

Thanks
(* I asked this also in WordPress answers, and asking here too)

Answer 1

REPLACE and INSERT ... ON DUPLICATE KEY UPDATE are not “relevant only for columns that are unique or defined as primary keys”, rather the determination of whether the record is new or already existing (and thus needs to be replaced/updated) is made on whether there is a unique key collision or not. As the MySQL documentation states:

MySQL uses the following algorithm for REPLACE (and LOAD DATA ... REPLACE):

1. Try to insert the new row into the table

2. While the insertion fails because a duplicate-key error occurs for a primary key or unique index:

   a. Delete from the table the conflicting row that has the duplicate key value

   b. Try again to insert the new row into the table

It describes INSERT ... ON DUPLICATE KEY UPDATE similarly.

Therefore, provided your REPLACE or INSERT ... ON DUPLICATE KEY UPDATE command identifies your updated records by their current primary keys, it should do exactly what you wish. According to the WordPress documentation, the primary key is the meta_id field, so with a SELECT you obtain this identifier for the existing cases and with an outer join, get NULL if it doesn’t already exist (which sets the column to the next auto_increment value):

REPLACE INTO `wp_postmeta` (
      `meta_id`
    , `post_id`
    , `meta_key`
    , `meta_value`
)
    SELECT
          `meta_id`
        , `post_id`
        , "key of interest"
        , "new value" -- assuming all posts to be set the same
    FROM
        `wp_postmeta`
        NATURAL RIGHT JOIN (
           SELECT 1 AS `post_id` -- where 1 is the first post_id
           UNION SELECT 2        -- 2 is the second
           UNION SELECT 3        -- 3 is the third
           -- etc.
        ) AS `temp_table`
    WHERE
        `meta_id` IS NULL OR `meta_key` = "key of interest"
;

If the new meta value is to be different for each post, you could add such values as a second column in temp_table:

REPLACE INTO `wp_postmeta` (
      `meta_id`
    , `post_id`
    , `meta_key`
    , `meta_value`
)
    SELECT
          `meta_id`
        , `post_id`
        , "key of interest"
        , `new_value`
    FROM
        `wp_postmeta`
        NATURAL RIGHT JOIN (
           SELECT 1 AS `post_id`, "foo" AS `new_value`
           UNION SELECT 2, "bar"
           UNION SELECT 3, "qux"
           -- etc.
        ) AS `temp_table`
    WHERE
        `meta_id` IS NULL OR `meta_key` = "key of interest"
;