mysql - 分组时如何与条件求和

Question

我有一个行列表，我想在分组时对行数求和(一种计数)，但仅当对于每一行，字段 customField=0 .

示例:

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

所以，输出应该是:

aaa 2
bbb 1
ccc 0

如何使用 MySql 实现？

编辑

事实上我真正的查询是这个:

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

只有当 forum_messages.original=0 时，我才必须计算 COUNT(forum_messages.id) AS total_replies，这就是我请求 SUM 的原因:)

Answer 1

只过滤结果集排除customField=1，即

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

结果:

aaa 2
bbb 1

显示所有标题，包括计数为 0 的标题:(如果需要其他计数也很有用)

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

结果:

aaa 2
bbb 1
ccc 0

更紧凑(但仅适用于 MYSQL):

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