fix https://github.com/FreshRSS/FreshRSS/issues/8252
regression from https://github.com/FreshRSS/FreshRSS/pull/6957
The current code works with MySQL 8+ (as well as PostgreSQL and SQLite), but not with MariaDB, because MariaDB does not support CTE (Common Table Expression) fully (which I was not aware of)...