MySQL

Differences between MySQL and MariaDB

• An official list, maintained by MariaDB, is located in their knowledgebase: Compatibility & Differences

SubSelects

MariaDB only sees references one level up from its current scope. For example, the following will work in MySQL, but not in MariaDB:

SELECT posts.*,
  (WITH c AS (
    SELECT id
      FROM conversions
     WHERE post_id = posts.id) -- Problem: posts is not visible here in MariaDB
  SELECT count(*) FROM c) as conversion_count
  FROM posts;

This query is functionally equivalent and compatible with both MariaDB and MySQL.

SELECT posts.*,
  (WITH c AS (
    SELECT id
      FROM conversions))
  SELECT count(*) FROM c WHERE post_id = posts.id) as conversion_count
  FROM posts;

This has come up in TryGhost/Ghost#15729 where the sub-sub-select has a UNION statement to combine the results of two tables together.