-- 运行此sql会得到一个null值，我们在做数据列表展示时，即使无数据也必须返回0而非null，  
如果我们不在后台代码里对运行sql之后的结果进行判断处理的话，  
我们就必须想办法在sql里解决这件事情。

select count(DISTINCT a.id)
FROM t_topic a
LEFT JOIN t_m_user b on b.id=a.create_by and b.deleted = '0'
LEFT JOIN t_dianzan c on c.topic_id=a.id AND c.deleted = '0'
LEFT JOIN t_topic_comment d on d.topic_id=a.id AND d.deleted = '0'
WHERE a.deleted = '0'
AND DATE_FORMAT(a.create_date,'%Y-%m-%d') = '2018-09-18'
GROUP BY a.id
ORDER BY a.create_date

-- 解决办法很简单 在外层嵌套一个count函数即可 
SELECT count(*) as total
FROM
(
	select count(DISTINCT a.id)
	FROM t_topic a
	LEFT JOIN t_m_user b on b.id=a.create_by and b.deleted = '0'
	LEFT JOIN t_dianzan c on c.topic_id=a.id AND c.deleted = '0'
	LEFT JOIN t_topic_comment d on d.topic_id=a.id AND d.deleted = '0'
	WHERE a.deleted = '0'
	AND DATE_FORMAT(a.create_date,'%Y-%m-%d') = '2018-09-18'
	GROUP BY a.id
	ORDER BY a.create_date
) t