请使用以下查询：

SELECT store_name
     , store_img
     , SUM(review_cnt) AS review_cnt
  FROM
     ( SELECT DISTINCT A.store_name
            , A.store_img
           , CASE WHEN B.store_idx IS NULL THEN 0 ELSE 1 END AS review_cnt
       FROM bookmark br
       JOIN board.store A 
            ON A.store_idx = br.store_idx
       LEFT 
           JOIN board.review B 
             ON A.store_idx = B.store_idx 
    WHERE br.user_id = ?
     )T

这个 WHERE 子句显然正在过滤掉所有行。对此我们无能为力。但你的查询也缺少一个 GROUP BY ，可以改进表别名，并且 join 情况不正确。
所以，试试这个版本：

select s.store_name, s.store_img, count(b.store_idx) as review_cnt 
from board.store s left join
     board.review r
     on s.store_idx = r.store_idx 
where b.store_idx in (select b.store_idx
                      from bookmark b
                      where b.user_id = ?
                     );