-- 執行此sql會得到一個null值，我們在做資料列表展示時，即使無資料也必須返回0而非null，  
如果我們不在後臺程式碼裡對執行sql之後的結果進行判斷處理的話，  
我們就必須想辦法在sql裡解決這件事情。

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
 

-- 解決辦法很簡單 在外層巢狀一個count函式即可 
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