mysql 一对多 group查询

场景:查询所有A表的数据，并且关联B表，再数据集中插入一个COUNT列。该列的数据是B表的COUNT

扩展：假设join的表数据为空，但我任然要把所有表A的数据取出来，那就用LEFT JOIN

SELECT 
        * 
FROM 
        question AS A
JOIN 
        (SELECT question_id, COUNT(*) AS COUNT FROM visitor GROUP BY  question_id) AS C 
  ON 
        A.id = C.question_id