mysql SQL_CALC_FOUND_ROWS加入问题

Question

为什么此查询返回0行？

SELECT SQL_CALC_FOUND_ROWS a.*, b.id AS key1, b.name AS catname 
FROM #__cd_ads a
LEFT JOIN #__cd_categories b ON a.type = b.id

删除b.id AS key1，b.name AS catname而不是作品。