如何在條件分組時總結條件

Question

我有一個行列表，並且我想在分組（COUNT類）時總結行數，但只有在每行的字段customField = 0。

例子：

title  customField 
aaa   1 
aaa   0 
bbb   0 
ccc   1 
bbb   1 
aaa   0 

因此，輸出應該是：

aaa 2 
bbb 1 
ccc 0 

我怎麼能與MySQL做呢？

編輯

其實我真正的查詢，這是一個：

SELECT forum_categories.title, COUNT(forum_topics.id) AS total_topics, COUNT(forum_messages.id) AS total_replies, MAX(forum_messages.date) AS last_message 
FROM forum_categories 
JOIN forum_topics ON forum_topics.category_id = forum_categories.id 
JOIN forum_messages ON forum_messages.topic_id = forum_topics.id 
GROUP BY forum_categories.id ORDER BY forum_categories.date 

我都數不過來COUNT(forum_messages.id) AS total_replies只有當forum_messages.original=0，這就是爲什麼我要求SUM :)

Answer 1

剛篩選結果集以排除customField=1，即

SELECT title, COUNT(*) AS numTitle 
    FROM yourtable 
    WHERE customField = 0 
GROUP BY title 
ORDER BY numTitle DESC 

結果：

aaa 2 
bbb 1 

要顯示所有冠軍，包括那些具有0計數：（也有用的，如果需要其他countings）

SELECT title 
     , SUM(CASE WHEN customField=0 THEN 1 ELSE 0 END) AS numTitle 
    FROM yourtable 
GROUP BY title 
ORDER BY numTitle DESC 

結果：

aaa 2 
bbb 1 
ccc 0 

更緊湊（但只適用於MYSQL）：

SELECT title 
     , SUM(customField=0) AS numTitle 
    FROM yourtable 
GROUP BY title 
ORDER BY numTitle DESC