The following query is currently not possible in MariaDB:

CREATE TABLE t (x int);

SELECT (

  SELECT u.x

  FROM (

    SELECT t.x

  ) u

)

FROM t;

The error I'm getting is:

SQL Error [1109] [42S02]: (conn:9) Unknown table 't' in field list

Most RDBMS, including recent versions of MySQL 8 now support correlating derived tables like that. It's an essential feature for more sophisticated queries.