如何使用MySQL中的子查询SELECT COUNT对表中的行进行计数？

Question

我在MySQL中运行此查询时收到错误Subquery returns more than 1 row。

SELECT a.*,
       (SELECT count(a.ads_id) FROM ads) as ads_count 
FROM ads a
INNER JOIN users u 
ON u.user_id=a.user_id 
GROUP BY u.user_id

我错过了什么？