MySQL：如何通过连接表计算文章数答案

【问题标题】：MySQL: how can I count number of articles by a join tableMySQL：如何通过连接表计算文章数
【发布时间】：2013-10-29 15:10:28
【问题描述】：

我有一个包含新闻项目的表，我还有另一个包含 media_types 的表，我想进行一个简单的查询，读取 media_types 表并计算每条记录有多少 news_items 存在。

结果将转换为我将用于图表的 json 响应，这是我的 SQL 语句

SELECT
  gc.country AS "country"
, COUNT(*) AS "online"
FROM default_news_items AS ni
JOIN default_news_item_country AS nic ON (nic.id = ni.country)
JOIN default_country AS c ON (nic.country = c.id)
JOIN default_geo_country AS gc ON (gc.id = c.geo_country)
LEFT JOIN default_medias ON (m.id = ni.media)
WHERE TRUE
  AND ni.deleted = 0
  AND ni.date_item > '2013-10-23'
  AND ni.date_item < '2013-10-29'
AND gc.country <> 'unknown'
AND m.media_type = '14'
GROUP BY gc.country
ORDER BY `online` desc LIMIT 10

这是我从 mysql 响应创建的 json 响应

   [
     {"country":"New Zealand","online":"7"},
     {"country":"Switzerland","online":"1"}
   ]

如何像这样将打印和社交数据添加到我的输出中

我希望 json 响应看起来像这样

   [
     {"country":"New Zealand","online":"7", "social":"17", "print":"2"},
     {"country":"Switzerland","online":"1", "social":"7", "print":"1"}
   ]

我可以在 select 语句中使用count (*) 来做这样的事情吗

COUNT( * ) as online, COUNT( * ) as social, COUNT( * ) as print

是否有可能或者我必须执行几个 SQL 语句来获取我正在寻找的数据？

【问题讨论】：

标签： mysql sql json

【解决方案1】：

这是一般结构：

SELECT default_geo_country.country as country,
       SUM(default_medias.media_type = 14) as online,
       SUM(default_medias.media_type = XX) as social,
       SUM(default_medias.media_type = YY) as print
FROM ...
JOIN ...
WHERE ...
GROUP BY country

【讨论】：

【解决方案2】：

我认为您需要条件聚合。但是，您的问题仅显示在线媒体类型。

通过使用表别名并删除反引号，您的查询将更具可读性。此外，如果media_type 是一个整数，那么您不应该将用于比较的常量用单引号括起来——我首先发现将字符串常量与整数列进行比较会产生误导。

我怀疑这是你想走的路。 . . . 所在的位置，您要填写其他媒体类型的计数。

SELECT default_geo_country.country as country,
       sum(media_type = '14') as online,           
       sum(default_medias.media_type = XX) as social,
       sum(default_medias.media_type = YY) as print
       . . . 
FROM default_news_items ni JOIN
     default_news_item_country nic
     ON nic.id = ni.country JOIN
     default_country dc 
     ON nic.country = dc.id JOIN
     default_geo_country gc
     ON gc.id = dc.geo_country LEFT JOIN
     default_medias dm
     ON dm.id = dni.media
WHERE ni.deleted = '0' 
 AND ni.date_item > '2013-10-23' 
 AND ni.date_item < '2013-10-29' 
 AND gc.country <> 'unknown' 
GROUP BY gc.country
ORDER BY online desc
LIMIT 10

【讨论】：