Question

I am creating a multisite CMS with codeigniter and am having trouble with a query for navigation links. Each page can have meta values that override the defaults (location_id = 0) with ones specific to the site if set. I have two tables cms_pages and cms_page_meta storing the information. Below is the relevant data

Table cms_pages
    page_id


Table cms_page_meta
    meta_id
    location_id
    page_id
    tag_name
    tag_value

I would like to do this in one call if feasible.

This is the call I was using, it causes pages to be listed twice if a title value exists for all locations (location_id = 0) and a specific location (location_id = x) where I just want the specific location if it exists otherwise the generic, otherwise NULL

SELECT cms_page.*, cms_page_meta.tag_value
FROM cms_pages
LEFT JOIN cms_page_meta ON cms_page_meta.page_id = cms_pages.page_id
WHERE cms_page_meta.location_id IN (0, x)
ORDER BY cms_pages.page_order`

I’m pretty sure I should to use coalesce but just can’t figure out how to implement it. Am I trying to do too much at once? Should I split the calls and process with PHP array_merge? I’d like to avoid that as I can cache the call and delete/regenerate the cache whenever I make a change to the pages or metadata. The server gets ~10,000 hits a day so speed is helpful but not necessary yet…

I changed the above query so it only grabs generic data for right now. Any help is greatly appreciated!

Answer 1

I think you should use something like this:

SELECT
  cms_page.*,
  COALESCE(cms_pm2.meta_id, cms_pm1.meta_id) as meta_id,
  COALESCE(cms_pm2.tag_name, cms_pm1.tag_name) as tag_name,
  COALESCE(cms_pm2.tag_value, cms_pm1.tag_value) as tag_value
FROM
  cms_pages LEFT JOIN cms_page_meta cms_pm1
  ON cms_pm1.page_id = cms_pages.page_id
     AND cms_pm1.location_id = 0
  LEFT JOIN cms_page_meta cms_pm2
  ON cms_pm2.page_id = cms_pages.page_id
     AND cms_pm2.location_id = x      
ORDER BY cms_pages.page_order